p-Nitrophenol, the main metabolite of parathion metabolism can be quantified by high performance liquid chromatography (HPLC) following very simple preparation of small quantities of blood the urine. Determination is carried out by UV detection at 315 nm following separation on a RP 18 column. Detection limits are 5-10 ng/ml serum and 50-100 ng/ml urine. levels of p-nitrophenol in blood and urine after survival from parathion intoxication are demonstrated.