When adult Richardson's ground squirrels, born in the wild, are kept in the laboratory under light:dark cycles of 14:10 (lights out at 2000 h), pineal N-acetyltransferase (NAT) activity and melatonin levels begin to rise shortly after lights out; both reach peak levels by 4 h into the dark phase. The rise in pineal NAT activity is partially blocked by phentolamine, an alpha-receptor blocker, and totally blocked by propranolol, a beta-receptor blocker. Phentolamine had no inhibitory influence on the nocturnal rise in pineal melatonin while propranolol partially prevented the increase in melatonin. Isoproterenol, a beta-receptor agonist, seemed to stimulate both NAT activity and the melatonin content in the pineal gland of the Richardson's ground squirrel. In the final study adult squirrels were kept in either 10 h of darkness at night or they were exposed to light (intensity = 370-400 ftc) during the normal period of darkness. Groups of squirrels were killed at 2000, 2400, 0200, 0400 and 0800 h. Darkness was associated with a marked rise in both pineal NAT activity and melatonin content. However, even when the squirrels were kept in light during the normal dark period, the activity of the acetylating enzyme and melatonin levels rose greatly and reached peak values at 0400 h.