The purpose of these experiments was to compare the behavioural and morphological effects of exogenous sex hormones in gonadectomized quail (Coturnix coturnix japonica) with those in quail having regressed gonads as a result of exposure to short days. In Expt 1, male quail were assigned to one of three treatment groups: intact, exposed to 16 h light:8 h darkness and injected with oil (group 16L); gonadectomized, exposed to 16 h light:8 h darkness and injected with 2.5 mg testosterone propionate (TP)/day (group 16L castrated); and intact, exposed to 8 h light:16 h darkness, and injected with 2.5 mg TP/day (group 8L). Groups 16L castrated and 8L responded similarly to testosterone, copulating with equal frequency and rapidity after the same number of days of treatment, and also developing proctodeal (foam) glands of a similar size. Only on day 7 of testosterone treatment did the results for these two groups differ. By day 14, the behaviour of both groups resembled that of the 16L birds. In Expt 2 female quail were assigned to the same three treatment groups, except that the hormone treatment was 25 μg oestradiol benzoate/day. Group 8L became sexually receptive sooner than the 16L ovariectomized quail, but by day 13 both groups had oviducts of similar size, were equally receptive, and were as receptive as the 16L females. The results suggest that the effects of photoperiod on sexual behaviour in this species are mediated largely, if not wholly, by the gonads. They also suggest that exposure to short days and surgical gonadectomy are rather similar experimental procedures in the quail.