This study has concentrated on the effects of an ovalbumin conjugated GnRH vaccine on the testicular function of colts, vaccinated at various ages from pre-pubertal to 3-yr old using three forms of the vaccine: mineral oil-based, water-based and subcutaneous implant. A preliminary investigation was also conducted on the ovarian function of 2-yr old fillies vaccinated with a water soluble GnRH vaccine. The research was aimed at exploring the potential benefits of these vaccmes m achieving reversible "immunocastration and immunospaying" in colts and fillies. The effects of these vaccines on the testicular function of the colts were monitored by measurement of testicular dimensions, plasma testosterone and androstenedione concentrations and GnRH antibody titres. Sexual behaviour, semen characteristics, photomicrographs and morphometric analysis of the testis and daily sperm production rates by the testes were also evaluated. The effects of a water soluble GnRH vaccine in the fillies were evaluated by ovarian and follicular palpations, ultrasonic examination and determining plasma progesterone and androstenedione concentrations and GnRH antibody titres. Antibody titres to GnRH greater than 1:1000 have been defined as being effective titres for suppressing testicular and ovarian function. The duration for which effective antibody titres were maintained after vaccination was defined as the immunosuppression period. The subcutaneous route of injection of the oil-based and the subcutaneous implant form of the GnRH vaccine caused tissue reaction at the injection site. The water soluble GnRH vaccine caused less reaction at the injection site and was much less harmful to the tissues than the oil-based vaccine. With a primary and a booster vaccination, effective antibody titres were produced for 27 to 32 weeks (intramuscular injection group) and 19 to 28 weeks (subcutaneous injection group) using an oil-based vaccine in pre-pubertal colts. The implant form of the vaccine produced effective antibody titres for 16 to 20 weeks with an equal number of injections. The pre-pubertal colts maintained effective titres for 24 to 37 weeks using 1 to 2 booster injections. Using two dose rates of the water soluble GnRH vaccine in 2-yr old colts, the 200 mg dose required 2 boosters while the 400 mg dose required 1 to 2 boosters to maintain immunosuppression periods of 27 and 29.8 weeks respectively. The immune response was highly variable in the colts vaccinated with water soluble GnRH vaccine but the fillies produced a more uniform immune response and maintained effective titres for 15 to 17 weeks with a primary and a booster vaccination. It was concluded that 2 booster vaccinations were required when using the 400 mg dose of the implant and water-based GnRH vaccine to produce an immunosuppression period equivalent to that produced by mineral oil-based GnRH vaccine. There was no significant (P>0.05) effect of these vaccines on the body weight and height of the colts and nor was there any effect on the body weight of the fillies. The major effects of these vaccines on the reproductive physiology of the colts were testicular atrophy, suppressed plasma testosterone (<0.20 ng/ml) and androstenedione secretions (<0.20 ng/ml). Testicular development was suppressed in the pre-pubertal colts, delaying puberty by approximately one year. The overall testicular width of the colts immunized with the 200 mg dose of water soluble vaccine was significantly greater (P<0.05) than those that received the 400 mg dose. There were no significant differences between the dose rate groups in plasma· testosterone and androstenedione concentrations. Sexual behaviour studies and semen collection attempts made during the immunosuppression period of the post-pubertal colts showed poor libido, failure to collect semen or collection of watery semen samples (accessory fluid) that were azoospermic or had very small numbers of sperm that were not suitable for normal fertility. Testicular histology in the yearling and 3-yr old colts during the immunosuppression period revealed significantly (P<0.05) lower seminiferous tubule volumes (ml), inter-tubular space volumes (ml) and Leydig cell volumes (ml) in the GnRH immunized colts when compared with the controls, with no significant age interactions. The daily sperm production per testis and per gm of testis in GnRH immunized colts was nil while the 3-yr old control colts had a significantly greater daily sperm production than the control yearling colts. Reversibility of the testicular function after vaccination occurred once the GnRH antibody titres fell below I:1000. This was accompanied by rising plasma testosterone and androstenedione concentrations, testicular growth and increased production of total number of sperm per ejaculate. The total number of sperm/ejaculate required to achieve normal fertility could not be obtained from the GnRH immunized pre-pubertal and 2-yr old colts at 216 and 121 days, respectively, after their antibody titres fell below 1:1000. The water soluble GnRH immunized pre-pubertal colts (Experiment 2.0) and 2-yr old colts (Experiment 4) took 216 to 300 days and 121 to 140 days after the antibody titres fell below 1:1000 to produce 3,336 and 5,570 x 106 total number of sperm per ejaculate respectively. Complete recovery from the effects of the vaccine could not be ascertained, due to the lack of control animals in both these experiments. Light micrographs of the testis obtained after surgical castration at the conclusion of the Experiment 4 showed 7.21 % (400 mg dose group) to 10.17 % (200 mg dose group) of seminiferous tubules in stage VIII of spermatogenesis and the presence of apparently normal Leydig cells. Long term effects of the implant form of GnRH vaccine (Experiment 3) indicated that 2 of 4 colts had recovered to a certain degree while two other colts were in the initial stages of recovery. The GnRH immunized fillies were in anoestrus during the breeding season while the control fillies displayed normal oestrous cycles and underwent 6 to 7 ovulations during the breeding season. The anoestrus condition was characterized by resistance to an approach by a stallion, basal plasma progesterone concentrations (<1.0 ng/ml), ovaries devoid of corpora lutea, follicular atresia, ovarian atrophy and a flabby and atonic uterus. The ovarian length and progesterone concentrations were significantly (P<0.05) greater in the 200 mg dose group when compared with those of the 400 mg dose group while the control fillies had significantly (P<0.05) greater ovarian lengths and progesterone concentrations than both the vaccinated groups. Reversibility of ovarian function in the GnRH immunized fillies was associated with resumption of normal oestrous behaviour, oestrous cycles, significant increases in progesterone concentrations, pregnancy and the delivery of normal foals. The GnRH immunized, fillies 200 mg dose group and 400 mg dose group, ovulated 30.8 ± 4.6 to 42.5 ± 10.6 weeks, respectively, after the primary vaccination while the control fillies ovulated 2.0 ± 1.0 weeks after the treatment groups received their primary vaccination. This study demonstrated that a safe and effective means of reversible "immunocastration and immunospaying" can be achieved in colts and fillies by using either 2.5 mg ovalbumin conjugated or 400 mg water soluble GnRH vaccine administered intramuscularly.