Procainamide (Pa) and its active metabolite--N-acetylprocainamide (NAPA)--pharmacokinetics was studied in 12 healthy volunteers in relation to acetylation phenotype. Acetylation phenotype was determined with sulphadimidine test. Blood serum levels of PA and NAPA were determined 0.5; 1.0; 1.5; 2.0; 3.0; 4.0; 8.0; and 12 hours following a single oral dose of 500 mg. Blood levels of both PA and NAPA were assayed with immunofluorescence polarization technique (FPIA), using TDx apparatus manufactured by Abbott. Pharmacokinetic parameters were calculated with the aid of pharmacokinetics independent of the model principles. All results were analysed statistically (AWOA). It was found that PA and NAPA pharmacokinetics depends on acetylation phenotype. Blood serum PA levels were higher in slow acetylators during the whole follow-up whereas NAPA levels were lower. Blood serum PA levels in rapid acetylators were decreased while NAPA levels were increased. Acetylation phenotype determined in sulphadimidine test confirmed bimodal procainamide acetylation.