A single injection of either 5 or 10 mg/kg 8-methoxypsoralen (8-MOP) was given intraperitoneally to male rats at the end of the 14 h light phase (at 2000 h). Two h later (at 2200 h), when the normal nocturnal surge of N-acetyltransferase (NAT) activity and melatonin content in the pineal gland had begun in vehicle-injected controls, mean pineal NAT after the 10 mg/kg 8-MOP was 1.8-fold higher than that after vehicle, though pineal melatonin content did not differ between vehicle- and drug-injected rats. By 4 h into the dark period (at 2400 h), the NAT activity in both 8-MOP injected groups of rats was greater than that in vehicle treated animals; again, however, 8-MOP treatment did not influence the pineal melatonin content. At 0200 h (6 h into the dark period), the difference between the NAT activity in pineals of rats treated with 5 mg/kg 8-MOP and the vehicle was not statistically significant, but the animals that received 10 mg/kg drug still had statistically elevated levels of the serotonin acetylating enzyme. At 0200 h the pineal melatonin levels were equivalent among the three treatment groups. Rats given 5mg/kg 8-MOP always had NAT values intermediate between those of rats injected with vehicle and those that received 10 mg/kg 8-MOP suggesting that the NAT response to the drug was dose related. These results show that the pineal response to psoralen involves an elevation of NAT activity without a commensurate change in the melatonin content of the gland.