Four mature Soay rams, cranially sympathectomized by removal of the superior cervical ganglia, were housed alongside four normal rams controlled lighting conditions of alternating 16 week periods of short days of 8 h light: 16 h darkness (8L : 16D) and long days (16L : 8D). The changes in the concentration of FSH, LH, prolactin and testosterone in the plasma, the size of the testes, the intensity of the sexual flush and the sexual and aggressive behaviour of the animals were recorded. While the control rams were able to respond to the artificial lighting conditions with synchronized cycles of reproductive activity, the ganglionectomized animals failed to respond. The treated rams had well-developed testes and relatively high levels of gonadotrophins and testosterone in the blood throughout the experiment. It is concluded that the cranial sympathetic nervous system is involved in the photoperiodic control of seasonal breeding in the ram, probably through its role in the innervation of the pineal gland.