Six rams of an ancient breed of domesticated sheep (SOAY) were subjected to an artificial light régime of alternating periods of long days (16 h light: 8 h darkness) and short days (8 h light: 16 h darkness) which induced seasonal development and regression of the testes during a period of 36 weeks. Over 2000 blood samples were taken, and the changes in plasma levels of FSH, LH and testosterone were related to the cycle of testicular activity. During long days plasma levels of gonadotrophins became very low and the testes regressed to about 20% of their maximum size; there was a corresponding reduction in plasma testosterone levels. When the rams were returned to short days reproductive development was again stimulated after 2-3 weeks with a progressive increase in plasma FSH and LH levels and consequent hypertrophy of the testes. It took about 16 weeks of short days for testicular activity to become maximal. Blood samples collected at hourly intervals for 24 h on ten occasions during the study revealed transitory peaks in plasma FSH and LH levels indicative of episodic release. Changes in gonadotrophin secretion were modulated primarily by alterations in the frequency of episodic releas; less than 1 spike per 24 h during long days increased to a maximum of 10 spikes/24 h under short daylengths. The peaks of FSH release were of smaller amplitude than those of LH, although during periods of frequent episodic release basal levels of fsh were increased to a greater extent than those of LH. A circadian rhythm was observed in the plasma levels of FSH, LH and testosterone, which was related to increased gonadotrophin release during the dark phase of the 24 h cycle; changes in blood haematocrit were also observed. The circadian changes appeared to be correlated with the activity cycle of the animals which in turn was dictated by daylight. A possible interrelationship between the circadian cycle and the seasonal cycle is discussed.