Examination of urinary metabolites of 32P- and 35S-parathion orally administered to rats confirmed that initial metabolism of parathion involved activation to paraoxon and degradation to diethyl phosphorothioic acid. Sulfur detached in the activation reaction was excreted as inorganic sulfate. Experiments in vitro with 35S-parathion indicated that the aryl-phosphate cleavage of parathion was mostly effected by liver microsomal oxidases; a minor portion was catalyzed by nonoxidative soluble enzymes requiring reduced glutathione. © 1969.