Three breeds of sheep (Dorset, Leicester, and Suffolk) and a crossbred line from these breeds (DLS) were observed throughout 1 yr to determine their sexual activity by daily exposure of the ewes to vasectomized rams. The DLS and Dorset ewes exhibited their first estrus earlier in the year (P < 0.01) and stopped cycling later than the Leicester and Suffolk ewes (P < 0.01). The duration of the breeding season in the DLS ewes was 20 days (P > 0.05) longer than in the Dorsets and 70 and 115 days (P < 0.01) longer than in the Leicester and Suffolk ewes, respectively. The percentage of Dorset and DLS ewes showing one or more estrous periods increased progressively from June to reach 100% in September for both breeds. The Suffolk and Leicester ewes commenced to cycle in late August and by September almost all ewes had shown an estrous period. The progressive decline in estrous activity became evident first with the Suffolk in January, then with the Dorset and Leicester in February and with the DLS in March. The Dorset, DLS and Leicester ewes had longer estrous cycles than the Suffolk ewes. Within ewes, length of estrous cycles was highly repeatable for all breeds. False estrus and silent estrus occurred in 1.0 and 3.9% of the estrous periods, respectively. Over successive estrous cycles, length of estrous cycles increased significantly.