A herd of 73 dairy cows was observed at pasture and during parlour milking for three weeks during the breeding season. Thirty-nine cows were seen in oestrus, which occurred either naturally or following prostaglandin induction. Readiness to enter the parlour, restlessness and elimination in the parlour were not found to be significantly affected by oestrus. However, cows in oestrus were more likely to be two or more batches away from their normally preferred parlour entry batch, and were more often early than late. Much more reliably indicative of oestrus, however, were large fluctuations in milk yield. Many cows showed a suppression at oestrus onset, with a rebound enhancement at the next milking. If a cow produced less than 75 per cent of its usual yield, there was a 50 per cent chance that it was in oestrus, and the rare occasions on which it gave 25 per cent more milk than normal only occurred during oestrus. Of seven cows induced but giving no behavioural indication of oestrus, six showed fluctuations in yield of this magnitude during the two days in which oestrus was expected.