Rat and hamster pineal glands were used in binding studies to characterize their β-adrenoceptors with a new specific antagonist ligand, iodocyanopindolol. The receptors were saturable, and the ligand was selective and demonstrated stereospecificity for both species. The rat pineal had a 20-fold greater density of β-adrenoceptors, while the affinity was one-third that of the hamster pineal. Utilizing this radioligand, we examined the effects of decreased sympathetic input to the pineal on β-adrenergic receptors in both species. Decreased noradrenergic input to the pineal gland of the hamster was accomplished by superior cervical ganglionectomy, or by exposing the animals to continuous light for 36 hours. Parallel studies were conducted with hamster pineal gland in which catecholamine synthesis was measured. The results indicate that a selective decrease in catecholamine synthesis in the hamster pineal does not change the β-adrenoceptor density or affinity. In contrast, a concomitant increase in β-adrenoceptor density but not affinity occurs in the rat pineal gland after similar decreased sympathetic input.