Derivatization of hydralazine [an antihypertensive agent] in plasma under acidic conditions yields values for the concentrations of apparent hydralazine that are dependent on the pH in the derivatization step. The existence of acid-hydrolyzable conjugates of unknown structure were postulated. These metabolites appear to be present in rat and human plasma after oral dosing. Procedures were developed for the selective analysis of hydralazine or its acid-labile conjugates, and applied to measurements of plasma concentrations in rat and man.