1. The reproductive performance of 128 first-lambing ewes born during 1966–71, and of 118 ewes kept on a twice-yearly lambing system during 1968–1972/73, in a commercial flock of Finnish Land-race sheep in Finland, was analysed.
2. The first-lambing ewes were exposed to the ram throughout the year, except in June and July. Out of 95 ewes born between January and May, 92 (97%) lambed at the age of 376·4 ± 29·9 days, and out of 20 born between July and September, 15 (75%) lambed when 323·8 ± 50·4 days old.
3. The average litter sizes at first lambing were 1·6, 1·9 and 2·6 for ewes with average ages of 324, 388 and 534 days, respectively; the 5-mo litter weights averaged 41·7, 45·7 and 70·5 kg per ewe for the three groups.
4. In the twice-yearly lambing system 55% of ewes lambed once within a calendar year and 45% lambed twice. The number of consecutive lambings and the number of lambs born per ewe per year were 1·52 ± 0·29 and 4·03 ± 1·18, respectively. The consecutive lambings of the 118 ewes, over a period of 1½ to 5½ yr, did not indicate any decline in reproductive performance.
5. The estimated lambing-to-conception interval in consecutive lambings was 116·6 ± 64·3 days; February had the lowest conception rate (0·7%) and November the highest (20·8%); April (16·5%) and September (15·5%) were similar.
6. Litter size was not affected by year, age of ewes, or lambing period. The mean lamb mortality up to 14 days of age, based on 1168 lambs born dead or alive, was 12·8% and the annual ewe wastage averaged 21·9%
7. For ewes which lambed twice within a calendar year, the lambing to conception interval was 71·7 ± 24·7 days. Such ewes produced 5·0±1·4 lambs per calendar year and the combined 5-mo weight of both litters was 168·2 ± 43·2 kg per ewe.