The reproductive performance of 5488 ewes in 34 commercially managed, lowland flocks in west Somerset was assessed by means of a questionnaire survey and flock inspections. The introduction of vasectomised rams before the breeding season (teasing) markedly affected the proportions of ewes mated in the first 16 days after the introduction of fertile rams. In teased flocks, 84.7 per cent (range 37.8 to 100 per cent) of the ewes were mated in the first 16 days, compared with 43-8 per cent (range 2.9 to 100 per cent) in unteased flocks (P<0.0001). Conception rates to first service and the overall percentage of ewes that conceived were similar in the teased and unteased flocks (89.3 per cent vs 92.6 per cent and 99.2 per cent vs 99.4 per cent, respectively). The overall mean (1.59) and range (1.31 to 2.04) litter size, mean (146 per cent) and range (123 to 196 per cent) lambing percentage and mean (94.6 per cent) and range (88.7 to 100 per cent) proportion of live lambs were similar in the teased and unteased flocks. The causes of reproductive failure were anoestrus (0.3 per cent), failure to conceive after multiple matings (0.3 per cent), visible abortion (2.4 per cent) and barren, that is, failing to lamb (3.4 per cent). The total reproductive loss rate was therefore 6.4 per cent of all ewes. Further non-reproductive losses occurred as a result of culling (0.3 per cent) and death during pregnancy (1.5 per cent).