The reproductive organs of the defassa waterbuck are similar to those of Caprinae. There appeared to be a seasonal fluctuation in testes weight, the lowest weight occurring in the driest month of the year. The female is poly-oestrous and monotocous and can conceive at the first post partum oestrus. Ninety-two per cent of implantations occurred in the right uterine horn but the left ovary was more active than the right. The highest incidence of births appeared to be correlated with the two wet seasons. The lowest incidence of conceptions appeared to be correlated with the lowest maximum temperatures. The possible relevance of temperature to sexual periodicity is discussed.