Urinary metabolites and biological half-life of chlorpyrifos (O, O-diethyl-O-3,5,6-trichloro-2-pyridinyl phosphorothioate) were investigated. Male Wistar rats weighing 200 g were intraperitoneally injected with chlorpyrifos at a level of 0.2 mmol/kg body weight. Both chlorpyrifos and 3,5,6-trichloro-2-pyridinol (TCP) levels in blood showed maximum values at 5 h post-injection, and then decreased rapidly. Biological half-lives of the blood chlorpyrifos and TCP were estimated to 8.15 and 24.66 h, respectively. Urine was collected for 96 h post-injection and hydrolyzed with 4 N HCl or beta-glucuronidase with sulfatase, and TCP released was determined. Urinary excretion levels of the acid hydrolysis-released TCP and the enzyme hydrolysis-released TCP accounted for 86 and 54% of chlorpyrifos administered, respectively. Urinary excretion levels of alkylphosphate for 96 h post-injection were analyzed. The excretion levels of diethylthiophosphate (DETP) and diethylphosphate (DEP) accounted for 45 and 15% of chlorpyrifos administered, respectively. These results indicate that 1) about half of the chlorpyrifos administered was directly hydrolyzed to DETP and TCP, 2) 10 to 20% was hydrolyzed to DEP and TCP after the oxidation to chlorpyrifos oxon, and 3) about 30% was dealkylated to TCP-phosphate after the oxidation.