One hundred and sixty adult ewes of the Chios breed were divided randomly into two groups. One group was used for breeding in spring (S) and the other in autumn (A). In both groups oestrous behaviour was detected once a day. Ovulatory activity, 7 d after oestrus, fertility, prolificacy, embryo mortality, birth weight of lambs and milk production were recorded. The percentage of ewes which exhibited oestrous behaviour was 92 and 100% in S and A groups, respectively, while fertility of mated ewes was 98 and 100%. Ovulation rate was significantly lower in S than in A ewes (M +/- sd; 2.85 +/- 1.07 versus 3.44 +/- 1.45, respectively, P < 0.0001). However, prolificacy did not differ between the two groups (2.04 +/- 0.77 versus 2.13 +/- 0.85, for S and A ewes, respectively) as it did not increase after an ovulation rate of 4.0. There were no differences in birth weights (except for triplets) but total milk production was significantly higher in S ewes (257.5 +/- 100.7 kg versus 153.0 +/- 51.9 kg, for S and A, respectively, P < 0.001). These results suggest that despite a higher ovulation rate in autumn, higher embryonic mortality at this time of the year leads to equivalent fertility, prolificacy and birth weight in spring as in autumn.