Sexual differentiation of copulatory behaviour in the male chick requires gonadal steroids.

1. Embryonic injections of 0.3 mg/egg of tamoxifen (TAM), 0.2 mg/egg CI-628 (both antioestrogens), 0.5 mg/egg (ATD (aromatisation inhibitor), or antibodies to oestradiol (E), all suppressed male copulatory activity (MCA) in young male chicks. 2. Embryonic injections with either flutamide (F, androgen antagonist) or high dose of antibodies to testosterone (T) only slightly suppressed MCA. 3. TAM had no effect on embryonic plasma LH levels, 24 and 48 h after injection. 4. It seems that at the embryonic stage oestradiol is required for the normal differentiation of MCA.