The seasonal reproductive cycle in rams was monitored in Mouflon (wild-type), Soay (feral type) and a variety of domesticated breeds of sheep (Shetland, Blackface, Herdwick, Norfolk, Wiltshire, Portland and Merino) by measuring the changes in the diameter of the testes (first three years of life in all breeds) and the blood plasma concentrations of FSH and testosterone (first four to six years of life in Mouflon and Soay rams). In the Mouflon rams there was a pronounced seasonal cycle in all the reproductive parameters from one to six years of age. The plasma concentrations of FSH increased from June to September associated with redevelopment of the testes; maximum testicular size and plasma concentrations of testosterone occurred in October at the onset of the rut. In the Soay and domesticated breeds the seasonal maximum in testicular size occurred in late September or October except in two of the southern breeds (Portland and Merino) which showed an earlier peak to the sexual cycle in August. The change in size of the testes from the minimum to the maximum was less in the domesticated breeds (135–157%) compared to the Soay (171%) and Mouflon (160%). Crossbred rams produced by mating Soay ewes (highly seasonal breed) with Portland or Merino rams (less seasonal breed) had a seasonal testicular cycle intermediate in timing compared to the pattern characteristics of the parent breeds; this is consistent with the involvement of multiple genes in the mechanism controlling the sexual cycle in the ram. The earlier seasonal onset of full testicular activity in the southern breeds is assumed to be the result of selection for a prolongation of the breeding season for early lambing.