Toxicokinetics of chlorpyrifos are being studied in F344 rats and B6C3F1 mice. Bioanalytical method for quantitation of chlorpyrifos and its main metabolite are being developed. Plasma concentrations of chlorpyrifos and its main metabolite will be determined after bolus iv injection of chlorpyrifos and gavage administration of chlorpyrifos. Bioavailability of chlorpyrifos will be estimated.