Daytime and nighttime urinary recovery as well as morning and evening trough serum levels of the antiarrhythmic agent tiracizine and three of its metabolites (M1, M2, and M3) were assessed during a 7-day multiple-dose study period (50 mg tiracizine twice daily) in eight healthy volunteers. A significantly lower mean steadystate urinary recovery was observed during the daytime as compared with during the nighttime (Ae_totrd = 42.0 ± 15.7% vs. Ae_totrn = 51.2 ± 19.6% This difference is mainly due to a substantial increase of M1 and a smaller increase of M2 urinary recovery by night. Results of additional in vitro investigations showed the ratio of the nonionic/ionic forms of M1 and M2 to be highly dependent on pH in the range of pH 5-pH 7. There-fore, the observed day-night variations might be attributed to alterations of ionization, i.e., nonionic tubular reabsorption of the metabolites due to circadian differences in urinary pH. Trough serum levels of M1 and M2 tended to be higher at 7 P.M. as compared with 7 A.M. Due to the narrow therapeutic index of class 1 antiarrhythmics, the present results indicate the need for further investigations concerning the effect of urinary pH on the pharmacokinetics of tiracizine and its metabolites.