Urine specimens of 8 organophosphate-poisoned patients on treatment with large doses of atropine were screened for atropine metabolites using thin-layer chromatography. Only atropine was detected. High performance liquid chromatography of urine specimens of a poisoned patient receiving large doses of atropine likewise detected only 3H-atropine after administration of 30 microCi 3H-atropine-sulphate. In the urine of a normal human volunteer treated with a single dose of 3H-atropine, 5 metabolic products were detected by this method. It is concluded that acute organophosphate-poisoning blocks atropine metabolism by inhibiting the hepatic microsomal enzymes.