Seasonal cycles were monitored in groups of wild (mouflon), feral (Soay) and domesticated breeds of sheep (Shetland, Blackface, Herdwick, Norfolk, Wiltshire, Portland, Merino, Soay x Portland and Soay x Merino) living outdoors near Edinburgh (56 degrees N). Changes in the blood plasma concentrations of prolactin and FSH, and growth of the horns and pelage were measured every half calendar month from 1 to 3 years of age. In all breeds there was a clearly defined seasonal cycle in the plasma concentration of prolactin with an 18-66-fold increase in mean values from the nadir in November and December to the peak in May and June. The seasonal increase in prolactin was closely correlated with the seasonal increase in the growth of the horns, both within and between breeds (e.g. time of peak prolactin vs horn growth for 11 breeds, R = 0.62, P less than 0.05). In the mouflon, Soay and some of the domesticated breeds of sheep (Wiltshire, Herdwick and Shetland), the seasonal increase in prolactin was also temporally correlated with the resurgence of growth of the pelage in spring and a conspicuous moult. In the other breeds developed to produce fine wool (e.g. Norfolk, Portland and Merino), there was no clear seasonal change in the pelage and growth continued throughout the year. Comparison between breeds indicated that continuous growth of the pelage was associated with higher plasma prolactin concentrations in winter. The times of the seasonal changes in plasma concentrations of prolactin were not significantly correlated with the corresponding changes in the plasma concentrations of FSH. The overall results are consistent with a role for prolactin related to the growth of the horns and pelage rather than the seasonal cycle in reproduction. The differences between the wild-type and the domesticated breeds in the pelage represent the effect of selective breeding to produce a long fine fleece which has involved changes in both the seasonal pattern of prolactin secretion and the growth characteristics of the hair fibres.