Behaviour in pigs is sexually dimorphic as early as 1 month of age; mounting of penmates is observed more frequently for males than for females. This mounting reaches its highest frequency during the 2nd month of life and then declines to a low frequency in prepubertal pigs. During the prepubertal period (3-5 months of age), bipotentiality of sexual behaviour is apparent in boars because they will not only mount oestrous females but they are also receptive to mounts by older, mature boars. If males are castrated during neonatal development (first 2 months of life) and treated acutely with oestrogen during adulthood, they display sexual behaviour that is characteristic of females; i.e. show a selective preference to remain near mature boars in a choice test, are receptive to mounts by mature boars, and have a short latency to receptivity after contact with a mature boar. Males that are castrated at 6 months of age or later, or males that are castrated neonatally and treated chronically with oestrogen or testosterone during the prepubertal period, display significantly less female behaviour after acute oestrogen treatment than do males castrated neonatally. Additionally, exposure of females to elevated testosterone during early fetal development results in no detectable changes in oestrous behaviour as adults. These observations support the hypothesis that defeminization of sexual behaviour in boars occurs as a result of elevated testicular steroids during pubertal development. The limited data available on masculine sexual behaviour in pigs indicate an activational role for gonadal steroids with little evidence for true masculinization per se. After prolonged testosterone treatment of mature females or males that are castrated before puberty, considerable courtship and mounting behaviours are exhibited by these individuals when placed with oestrous females. Studies have not evaluated differential sensitivity of such animals to dosage or duration of testosterone treatment. Differentiation of sexual behaviour in boars therefore involves primarily a loss of sensitivity to display female-typical behaviours. Presently, pigs differ from other mammals that have been investigated because sexual differentiation of reproductive behaviour occurs during pubertal development and not during gestation.