The testosterone response to gonadotropins in Nguni bulls with unilateral gonadal hypoplasia and aplasia was studied. Despite having one grossly affected or totally absent testis, the residual testicular tissue was found to be able to maintain a normal basal serum testosterone concentration, and to yield a normal response to PMSG with respect to peak testosterone value and peak response time. It is possible that this compensation is responsible for the maintenance of fertility of bulls with gonadal hypoplasia as observed by other workers.