The acute exposure of wild-captured Richardson's ground squirrels to fluorescent light (intensity = 370-400 ftc) at 2400h, 4 hours after the onset of darkness, was followed by a slight depression in the activity of pineal N-acetyltransferase (NAT); during the same 60 min period, pineal melatonin levels were not inhibited. Conversely, when laboratory-raised squirrels were either exposed to light at night or kept in their normal period of darkness, pineal NAT activity and melatonin levels differed greatly between the two groups. In darkness both NAT and melatonin rose sharply and remained elevated during most of the night. When animals were exposed to light at night the NAT rhythm was completely suppressed and the rise in melatonin was severely dampened. Finally, the administration of isoproterenol (6 mg/kg) increased otherwise low daytime levels of both NAT activity and melatonin levels in the pineal gland of the ground squirrel.