Suprachiasmatic lesions disrupt the daily rhythmicity in the sexual behaviour of normal male rats and of male rats treated neonatally with antioestrogen.

: Male rats were treated daily with oil or 100 micrograms of the antioestrogen, ethamoxytriphetol (MER-25), for the first 10 days of life and, when adult, lesions were made in the suprachiasmatic nuclei (SCN) of the hypothalamus or control lesions were made above the SCN and the rats were tested for sexual behaviour. Treatment with MER-25 enhanced the daily rhythmicity in both mounting and lordosis behaviour and SCN lesions disrupted these behavioural rhythms and the rhythm in the mounting behaviour of oil-treated rats. Rats treated with MER-25 and with SCN lesions showed high levels of mounting and lordosis behaviour throughout the light : darkness cycle. These results support the hypothesis that sexual differentiation by perinatal androgen stimulation uncouples the central rhythm generator from the neural substrates of sexual behaviour in rats.