PHOTOTRANSDUCTION in the vertebrate retina is dependent in part on a cyclic GMP-activated ionic channel in the plasma membrane of rods and cones1,2. But other vertebrate cells are also photosensitive. Cells of the chick pineal gland have a photosensitive circadian rhythm in melatonin secretion that persists in dissociated cell culture3,4. Exposure to light causes inhibition of melatonin secretion, and entrainment of the intrinsic circadian oscillator5,6. Chick pinealocytes express several 'retinal' proteins, including arrestin7, transducin8 and a protein similar to the visual pigment rhodopsin9. Pinealocytes of lower vertebrates display hyperpolarizing responses to brief pulses of light10,11. Thus it is possible that some of the mechanisms of phototransduction are similar in retinal and pineal photoreceptors. We report here the first recordings of cyclic GMP-activated channels in an extraretinal photoreceptor. Application of GMP, but not cyclic AMP, to excised inside-out patches caused activation of a 15-25 pS cationic channel. These channels may be essential for phototransduction in the chick pineal gland.