SUMMARY Three adult Suffolk rams were bled at weekly intervals for 14 months, and at hourly intervals for 24 h during the summer (one ram) and autumn (two rams). Luteinizing hormone (LH) was measured by radioimmunoassay, and testosterone by a competitive protein binding assay. There were episodic bursts of LH secretion during the 24 h period in both the summer and the autumn; the frequency of discharge was lower in the summer, but the amplitude (0·1–8·0 ng/ml) did not appear to change. It was not always possible to detect seasonal changes in LH concentration in single blood samples taken once a week throughout the year. The blood testosterone levels also showed marked fluctuations throughout the 24 h, and the frequency of the peaks was lower in the summer. But in contrast to LH, the magnitude of the testosterone peaks also changed throughout the year; from January to September the testosterone concentration ranged from 0·5 to 10 ng/ml plasma, whereas from October to December it ranged from 3 to 28 ng/ml. Thus in temperate regions the ram, like the ewe, shows seasonal changes in gonadal endocrine activity, although some degree of spermatogenesis continues throughout the whole year.