ABSTRACT Pfeiffer (1935, 1936) reported the induction of constant oestrus in female rats following the transplantation of testes from litter-mate males just after birth and noted that the ovaries of these animals did not contain corpora lutea. These changes remained after removal of the testis transplants. The same effects were obtained by Bradbury (1941) following the administration of multiple doses of testosterone propionate. Barraclough & Leathern (1954) found that a single injection of 1 mg of testosterone propionate at 5 days of age led to permanent sterility in female mice, with no corpus luteum formation in their ovaries. Similar results were obtained in rats by Barraclough (1961) with the administration of a single injection of 1·25 mg of testosterone propionate. This permanent change in ovarian function does not appear to be a direct effect upon the ovary (Bradbury, 1941). The masculinizing influence of testosterone propionate appears to be exerted upon the brain, with only an indirect effect upon the anterior pituitary gland by way of the hypothalamus (Adams Smith & Peng, 1966). The influence of the ovary in sexual differentiation of the central nervous system is not clear. This study was undertaken in an endeavour to determine the pattern of sexual maturation of the brain of the female rat ovariectomized at birth and treated with testosterone propionate at different times during the first three post-natal weeks. Different doses of testosterone were administered to see whether dosage influenced the receptive period of the animal’s brain. The receptive period found in ovariectomized animals would allow a comparison with results obtained by workers who had treated normal females with testosterone, so that an assessment could be made as to whether the presence of the ovary influenced the length of this receptive period. Control procedures were planned to allow an observation of the sexual maturation of the brain of ovariectomized females not subjected to androgen treatment.