Intracellular recordings from 103 photoreceptors in the excised pineal body of adult trouts were obtained by using single electrode current- and voltage-clamp techniques. The photoresponses to brief flashes showed the same polarity but a slower time course than those previously recorded from retinal photoreceptors of lower vertebrates. Pineal photoreceptors showed spectral sensitivity peaks at about 495 and 521 nm and absolute sensitivity comparable to retinal cone cells of the same species. The photoreceptor membrane conductance, measured under voltage clamp during moderate illumination was about 10% lower than in the dark, and the extrapolated reversal potential of the response was at 60 mV above the dark membrane potential. The addition of 3-isobutyl-1-methylxanthine (IBMX) to the perfusate was followed by a receptor depolarization in the dark and by a slow-down of the response kinetic. Pineal receptor cells produce constant amplitude responses during steady illumination, without displaying the delayed slow depolarization typically associated with light adaptation of retinal photoreceptors. Photoresponses to brief flashes superimposed on a steady illumination are decreased in amplitude by an amount directly related to the background intensity. Increase of the background intensity leads to threshold increments without significant changes of the saturation intensity, resulting in a gradual compression of the cell dynamic range. These results were discussed relative to light adaptation in retinal photoreceptors. The conclusion can be drawn that the response properties of pineal photoreceptors during steady illumination are part of an unknown, self-regulating mechanism to lock the rate of metabolism and secretion of indolamines to the absolute level of diurnal light.