Concentrations of fenbendazole and its sulfoxide, oxfendazole, and sulfone metabolites were determined in 6 sheep after oral administration of fenbendazole (10 mg/kd of body weight). Mean peak concentrations in plasma of fenbendazole, oxfendazole, and sulfone of 0.15, 0.29, and 0.17 micrograms/ml occurred 24, 30, and 36 hours after administration, respectively. Mean peak concentrations in abomasal fluid were 1.82, 0.66, and 0.07 micrograms/ml occurring at 30, 48, and 72 hours, respectively. Fenbendazole and oxfendhzole were detectable in plasma and abomasal fluids for 5 days after administration. Much of the anthelmintic activity of fenbendazole may be due to the oxfendazole metabolite. Plasma concentrations of fenbendazole were less and persisted for a shorter period after intra-abomasal administration than after oral administration.