Adenyl cyclase activity of rat pineal gland homogenates was determined by measuring the rate of conversion of adenosine triphosphate-8-C14 to radioactive cyclic 3’,5’-adenosine monophosphate. Norepinephrine and sodium fiuoride, which act at different sites on the pineal adenyl cyclase system, increased enzyme activity. Exposure of rats to continuous light for several days potentiated the norepinephrime-induced activation of pineal adenyl cyclase. Environmental lighting also enhanced the response of the pineal enzyme to sodium fluoride, indicating that light produced a general alteration of the enzyme system rather than a change at a site specific for the catecholamine. The light-induced increase in the activation of adenyl cyclase by norepinephrine was antagonized by removal of the superior cervical ganglia, a procedure which denervates the pineal gland. Effects similar to those produced by light exposure were seen in pineal glands chronically denervated (4-10 weeks) by bilateral superior cervical ganglionectomy ; that is, the stimulation of adenyl cyclase elicited by both norepinephrine and sodium fluoride was increased. In addition, the activation of adenyl cyclase by maximal as well as by submaximal concentrations of the catecholamine was potentiated by chronic denervation, indicating that there was more enzyme rather than a greater affinity of adenyl cyclase for norepinephrine. These results suggest that sympathetic nerve activity may have a chronic as well as an acute influence on the catalytic capacity of the pineal adenyl cyclase system.