Summary. Three groups of 5 prepubertally castrated boars received twice weekly subcutaneous injections from 16 to 28 weeks of age of 5α-dihydrotestosterone propionate (1 mg/kg), oestradiol-17β dipropionate (0·1 mg/kg), or a combination of the two steroids. The pigs were tested each week for sexual behaviour with an oestrogen-treated ovariectomized gilt. 5α-Dihydrotestosterone did not induce copulatory behaviour whereas oestradiol initiated mounting, although this behaviour was not maintained. However, the combined hormonal treatment induced the full pattern of copulation, including ejaculation, in 4/5 pigs. Oestrogen-treated animals also showed champing and salivation. None of the pigs treated with 5α-dihydrotestosterone showed lordosis when exposed to a mature boar. Increases in the size of the penis, the ischioand bulbocavernosus muscles, seminal vesicles and bulbourethral glands were produced by 5α-dihydrotestosterone, and, with the exception of the penis, this hypertrophy was further enhanced by additional treatment with oestradiol. Increases in the weight of the prostate and muscular urethra were due mainly to the effects of oestrogen; this steroid caused extensive development of fibromuscular tissue and also increased mammary teat length. In the submaxillary salivary glands, synergism between 5α-dihydrotestosterone and oestradiol resulted in hypertrophy of the serous cells.