Labeled compounds were used to study the metabolism of salicylates in rats. A method, electrophoretic separation, was developed to identify the oxidative metabolites. Analysis of tissue extracts by this method generally revealed only the administered compounds. However, gentisic acid was found in the pituitary region of fevered rats administered either acetylsalicylic or salicylic acid. It was not found in the pituitary after administration of these compounds to nonfevered rats or after administration of gentisic acid to fevered rats.