Secretion ofLH in rams exposed to two different photoperiods

In the mature ram pulses of luteinizing hormone (LH) are released from the anterior pituitary every few hours randomly throughout the day to appear in the peripheral circulation as transitory surges in the hormone concentration (Katongole, Naftolin & Short, 1974; Sanford, Winter, Palmer & Howland, 1974). Since the episodic secretory patterns recorded in the peripheral blood develop during puberty (Foster, 1974) and change in relation to the breeding season (Lincoln, 1976), they clearly have some bearing on the functional state of the testes. The purpose of the present study was to investigate the episodic nature of LH secretion in sheep exposed to different photoperiods. Rams of the Soay breed were selected since they have conspicuous seasonal changes in reproduc-tive physiology (Lincoln, 1976) and behaviour (Jewell, Milner & Morton Boyd, 1974). Six mature rams, 3-5 years of age and weighing on average 31\m=.\7kg, were housed in a light-proof building in individual pens, and a pelleted maintenance diet was given once daily at about 08.30 hours. Lighting was provided by 4 100 W white fluorescent strip lights (Crompton, U.K.) providing illumination of 140-180 lux at floor level. The normal cycle of daylength was replaced by a simplified regimen with alternating periods of long days (16 hr light: 8 hr dark, 16L: 8D) and short days (8L: 16D) each lasting 12-17 weeks. Only the timing of 'lights out ' was adjusted; 'lights-on ' occurred at 08.0