Eight adult Soay rams (four control and four cranially sympathectomized by removal of the superior cervical ganglia) were exposed to alternating 16-week periods of short days of 8 h light: 16 h darkness (8L : 16D) and long days (16L : 8D) for more than 3 years, and the changes in the diameter of the testes were recorded. Once during short days and once during long days blood samples were collected hourly for 28 h, and the concentrations of melatonin, prolactin, FSH, LH, testosterone, tri-iodothyronine (T₃), thyroxine (T₄) and cortisol were measured by radioimmunoassay. In the control rams the testes were reduced in size during long days compared to short days, and the blood concentrations (24-h mean values) of FSH, LH and testosterone were decreased. The levels of prolactin were much increased, while there was no significant change in the mean levels of melatonin, T₃, T₄ and cortisol. During both photoperiods there was considerable hour-to-hour variation in all eight hormones indicative of episodic secretion, as well as a consistent variation related to the time of day which was most pronounced for melatonin and T₃. There was a clear difference in the daily profile of plasma melatonin levels between short and long days. In the superior cervical ganglionectomized rams there were no significant changes in the size of the testes or in the hormone titres between short and long days. Compared to the controls the plasma levels of LH, FSH, testosterone and prolactin were in the intermediate range. Some consistent diurnal variation was evident in the levels of all the hormones measured, with a pattern similar to the controls for a few of the hormones (e.g. T₃) but quite different for others (e.g. melatonin).