Six adult Soay rams were subjected to an artificial lighting régime of alternating 4 month periods of long days (16 h light:8 h darkness) and short days (8 h light:16 h darkness) which induced a seasonal cycle in testicular development and regression during a period of 32 weeks. On 15 occasions during the study pituitary responsiveness was assessed by measuring the concentration of luteinizing hormone (LH) in jugular blood samples collected at frequent intervals after the intravenous injection of 1 microgram synthetic luteinizing hormone releasing hormone (LH-RH). It was shown that both the magnitude and duration of the induced release of LH changed in relation to the photoperiod; the magnitude was greatest during long days when the rams were sexually quiescent, while the duration of the LH release was greatest during short days at the peak of testicular activity. This pattern of responsiveness was modified by implantation of the rams with testosterone.