Circadian rhythms and environmental lighting regulate a number of endocrine and behavioral functions. The rat pineal gland provides an ideal model for the study of mechanisms which generate and control circadian rhythms. Evidence indicates that the neurotransmitter acetylcholine is involved in the central regulation of circadian rhythms by light. There is a circadian rhythm in the rat pineal in levels of the synaptic protein designated protein I, which is mediated by beta-adrenergic neurotransmission.