Relative bioavailability of a 100 mg tablet formulation of the antiarrhythmic agent tiracizine (CAS 78816-67-8) compared to a 50 mg formulation was assessed in a simple cross over study after single administration of a 100 mg dose to 12 healthy volunteers. Tiracizine and three of its metabolites (M1, M2 and M3) were measured in serum and urine by high pressure liquid chromatography. AUC (means after administration of the test preparation and 95% nonparametric confidence interval for the ratio test preparation/reference preparation) were 391.5 ng · h/ml and 0.87-1.11 for tiracizine, 5184.5 ng · h/ml and 0.94-1.26 for M1, and 1319.9 ng · h/ml and 0.88-1.16 for M2. Mean maximum serum concentrations after the test preparation and corresponding 95% confidence interval were 111.2 ng/ml and 0.86-1.20 for tiracizine, 301.2 ng/ml and 0.98-1.22 for M1, 54.6 ng/ml and 0.86-1.17 for M2, and 35.2 ng/ml and 0.82-1.17 for M3. t(max) did not differ after the two preparations for tiracizine, M2 and M2, but was significant lower for M1 after administration of the test preparation (2.2 ± 0.7 vs 3.0 ± 1.2 h). Total urinary recovery (sum of parent compound and metabolite recovery) up to 32 h after intake of the test preparation was 31.2% of the administered dose. The corresponding 95% confidence interval was 0.84-1.08. Statistical evaluation of all parameters revealed bioequivalence between the two preparations if a single dose of 100 mg is administered.