In order to elucidate whether magnetic field effects on mammalian pineal function are direct, or instead indirect via retinal disturbances, acutely blinded and intact male rats were subjected to a single nocturnal magnetic stimulus. Then pineal N-acetyltransferase activity and melatonin content were assayed. Only in intact animals did the magnetic stimulus significantly reduce pineal activity, i.e. no effects were detected in blinded animals. These data point to a retinal magnetosensitivity which may serve to modulate pineal gland function.