The tuberculostatic drug isoniazid is acetylated by pineal N-acetyl transferase (P-NAT) of the rat. Kinetics of isoniazid acetylation are different compared to that of serotonin acetylation. Apparent enzyme kinetic parameters for the unstimulated type of P-NAT were: Km: 4.6 mmol/l (serotonin) and 31.8 mmol/l (isoniazid), respectively, Vmax: 28.9 pmol/15 min (serotonin) and 336.7 pmol/15 min (isoniazid), respectively. The velocity of isoniazid acetylation is inversely correlated to the amount of enzyme preparation used for incubation. Therefore the existence of an endogenous inhibitor of P-NAT is proposed. Eadie-Scatchard plots of enzyme activities obtained with different amounts of homogenate and several concentrations of substrate indicate that in the diluted homogenate enzyme activity originates from more than one enzyme or an enzyme consisting of structurally different subunits.