Department of Physiology and Biophysics, Colorado State University, Fort Collins, Colorado 80523, U.S.A. (Received 13 September 1977) Mammalian pineal gland activity is controlled by environmental lighting schedules. Light exerts its influence via a neuronal pathway originating in the retina (Moore & Klein, 1974) and as a consequence of this photoperiodic control, the concentration of melatonin in the plasma is raised during periods of darkness and depressed during periods of light (Rollag & Niswender, 1976). The response of the pineal gland to photostimulation is surprisingly rapid. Within 5 min of a darkness to light transition, there is a precipitous decline in pineal N-acetyltransferase activity in the rat (Deguchi & Axelrod, 1972; Klein & Weller, 1972). In sheep, peripheral concentrations of melatonin decline within 5–10 min of a darkness to light transition (Rollag, O'Callaghan & Niswender, 1978). A circadian rhythm of blood flow to the pineal gland analogous to the rhythm of melatonin