Methyl-N[5 [[4-(2-pyridinyl)-1-piperazinyl]carbonyl]- 1H-benzimidazol-2-yl] carbamate (CDRI-81/470) is a broad spectrum anthelmintic agent, effective against both intestinal and systemic parasitism. Tissue distribution and excretion of CDR1-81/470 were studied in rats after a single oral dose of 100 mg kg(-1) CDRI-81/470. One of the metabolites was identified in pilot studies as its N-decarboxylate derivative and characterized by synthesis. HPLC assay methods for the simultaneous estimation of CDRI-81/470 and its N-decarboxylate derivative in tissues, bile, urine, and faeces were developed and validated. The parent compound was quantitated in all major tissues and organs up to 48 h post-dose. Among the tissues other than serum, the highest concentrations of CDRI-81/470 were found in liver, whereas only trace levels were found in brain. Approximately 3% of the administered dose was excreted unchanged in urine at 120 h postdose, whereas approximately 7% was recovered in faeces. The contribution of the biliary route for the excretion of parent compound was less than 0.5%. The N-decarboxylate derivative was quantitated in faeces (1-4%) and bile ( < 0.1%) but was absent in serum, tissues, and urine. An additional metabolite was isolated from bile and characterized as the pyridinyl-5-hydroxy derivative of CDRI-81/470. CDRI-81/470 showed rapid absorption and distribution into all major organs and tissues, and underwent extensive metabolism in rats. Two metabolites in bile were identified and characterized by synthesis.