Netobimin (NTB), a benzimidazole prodrug with a good anthelmintic spectrum, was administered orally to female rats at a dose of 59.5 mg NTB/kg, to study its pharmacokinetic behavior and the disposition of its most important metabolites, albendazole (ABZ), albendazole sulfoxide (ABZSO), and albendazole sulfone (ABZSO2). ABZ was found in plasma after 6 hr. Peak plasma concentrations (C(max)) and areas under curves (AUC) of ABZSO were eight- and fourfold higher, respectively, than those of ABZSO2. To study NTB disposition in pregnant rats, three different drug doses (50, 59.5, and 70.7 mg/kg) were given. No significant differences were found between plasma concentrations for each metabolite at the three doses studied. Only ABZ concentrations rose slightly as dose increased. ABZ, ABZSO, and ABZSO2 were found in amniotic sacs and embryos at concentrations that were higher than plasma at the same times. The fetuses obtained after administration of each of the doses of NTB were studied to detect developmental toxicity. A significant correlation was found between rate of developmental toxicity and metabolite concentration. ABZSO embryo concentrations could be the main factor accounting for toxicity.