Hydralazine is an antihypertensive vasodilator agent. Lack of specific assay techniques for its measurement have delayed elucidation of its pharmacokinetic profile. This study compares the plasma profiles of hydralazine, measured both by a specific and by a previously published nonspecific assay and of a major plasma metabolite, hydralazine pyruvic acid hydrazone. After po and iv administration of hydralazine, peak hydralazine levels were lower (7-33%) and plasma half lives were shorter (15-31%) when measured by the specific technique. The mean plasma half life of the pyruvic acid hydrazone was 156 min and mean urinary clearance, 28 ml/min. The plasma profile of hydralazine and of the major metabolite, the pyruvic acid hydrazone, do not appear to correspond to the duration of antihypertensive effect of administered hydralazine.