In order to determine whether the metabolism of the antiarrhythmic drug N-propylajmaline is under the same genetic control as sparteine metabolism, the pharmacokinetics of this antiarrhythmic drug were studied in a groups of six extensive and four poor metabolizers of sparteine. Pronounced differences in terminal half-life, total plasma clearance, metabolic clearance and urinary excretion of N-propylajmaline were observed between extensive and poor metabolizers. A close relationship between the total clearance and metabolic clearance of N-propylajmaline and sparteine could be demonstrated. Clinically available N-propylajmaline is a 55% to 45% mixture of the i- and n-diastereomers. The extensive metabolizers exhibited stereoselective metabolism; the i-diastereomer was preferentially metabolized. Poor metabolizers were characterized by a loss of this stereoselective metabolism. Five subjects were treated for 7 days with a daily N-propylajmaline dosage of either 60 mg or 20 mg. Since a close relationship between the clearance of N-propylajmaline and the metabolic ratio of sparteine had been observed after single dosing the metabolic ratio of sparteine was used to predict N-propylajmaline steady-state plasma concentrations during multiple dosing. Only in two extensive metabolizers with a metabolic ratio less than 0.4 predicted and observed, steady-state plasma concentrations were in good agreement. In the other three subjects observed steady-state plasma concentrations were appreciably higher than predicted. In these three subjects metabolic N-propylajmaline clearance decreased indicating saturation N-propylajmaline metabolism during multiple dosing. The data indicate that N-propylajmaline metabolism is subject to a genetic polymorphism controlled by the sparteine/debrisoquine gene locus.