A seasonal effect was demonstrated on the occurrence of oestrus and on the length of oestrous cycles in Bunaji and Bokoloji cows. There was a gradual lengthening of oestrous cycle which resulted in fewer cycles occurring in the dry and pre-rainy seasons. Oestrous cycle length was the same for Bunaji (22.89 +/-0.70 days) and Bokoloji (23.76 +/- 0.65 days) cows (P less than 0.05). Season had an equally depressing effect on the duration and intensity of oestrus in both breeds of cows. During the dry and pre-rainy seasons the behavioural signs of oestrus were poorly manifested and lasted for only a short period. During the rainy and pre-dry seasons, the duration of oestrus and behavioural signs were much more pronounced.