Six castrated Soay rams were given testosterone implants, and exposed to an artificial lighting regimen of alternating 16-week periods of long days (16L:8D) and short days (8L:16D) for over 1 year. Under these conditions, the blood plasma concentration of testosterone was maintained relatively constant (4.3--6.4 ng/ml), while the levels of LH and prolactin varied in relation to the alterations in the photoperiod; low levels of LH occurred during long days when the rams were hyperprolactinaemic. During the last period of exposure to long days, 4 of the rams were cranially sympathectomized by removing the superior cervical ganglia. The effect of long daylengths on the secretion of LH and prolactin was blocked; prolactin concentrations began to decline after 2 weeks and LH levels increased. Ganglionectomy also influenced the plasma levels of melatonin; after the operation there was no longer a consistent 24-h rhythm in melatonin values and the increase associated with darkness was abolished. This results supports the view that the regulation of melatonin secretion is involved in the photoperiodic control of reproduction in rams.