Urinary valproic acid (VPA) and VPA metabolite profiles were determined before (day 1) and after (day 2) the administration of antipyretic doses of acetylsalicylic acid (ASA) to seven subjects with steady-state levels of VPA. Of the 13 metabolites assayed by GC/MS, levels of (E)-2-ene VPA and 3-keto VPA were significantly decreased on day 2, whereas those of the VPA conjugates (glucuronide) and 4-ene VPA were significantly increased. The beta-oxidation pathway consisting of (E)-2-ene VPA, 3-OH VPA, and 3-keto VPA was decreased from 24.5% +/- 10.3% of total metabolites excreted on day 1 to 8.3% +/- 4.2% on day 2, a decrease of 66% (P less than 0.05). VPA glucuronide content increased from 50.5% +/- 12.6% on day 1 to 65.5% +/- 14% of total excreted on day 2, an increase of 30% (P less than 0.05). The day 2/day 1 ratios of VPA glucuronide correlated significantly with the day 2/day 1 ratios of VPA mean free fraction (r = 0.9424; P = 0.005) in six of the seven subjects. Inhibition of VPA beta-oxidation by salicylate was sufficient to counterbalance the increased elimination of VPA as its conjugates and explains why total clearance of VPA after salicylate remains unchanged even though the free fraction of VPA is increased. Metabolic profiles indicate that salicylate likely inhibits VPA beta-oxidation by reducing valproyl-coenzyme A formation.