After injection of 20 mg/kg of the monoamine oxidase inhibitor pargyline every other day up to 9 days, the activity of N-acetyltransferase (NAT) in the pineal gland of rats remained unchanged during the light-period. At the darkness, however, an increase above controls was observed. The latter effect was not affected by subchronic administration. The monoamine oxidase inhibitor activated not only the NAT but a second enzyme with different kinetic parameters. The drug levelled the circadian changes in the content of serotonin whereas noradrenaline and 5-hydroxyindole acetic acid were hardly influenced. These findings may be explained in such a way that pargyline activates the enzyme not via beta-receptors as supposed by Deguchi and Axelrod (1972a) but by an intracellular mechanism. The increased serotonin concentration might be responsible for the additional activation of the enzyme.