Pregnant ewes were implanted with 1 g testosterone between Days 30-80, 50-100, 70-120 or 90-140 of gestation. Treatments which began on Days 30, 50 or 70 resulted in the birth of androgenized females which failed to show regular oestrous cycles in adult life, but which exhibited patterns of male like behaviour. This was most marked in the Day 50-100 and 70-120 groups, whereas complete masculinization of the external genitalia was confined to the Day 30-80 group. Animals in the Day 90-140 group had regular oestrous cycles although they showed slight enhancement of masculine behaviour compared to the control ewes. These results demonstrate that androgenization involves both a suppression of female behavioural patterns, and the development of male patterns; these are not mutually exclusive.