This study was performed in order to determine the effects of valproic acid on serum free carnitine, beta-hydroxybutyrate and blood amamonia. Serum free carnitine, beta-hydroxyhutyrate and blood ammonia levels were measured in 24 epileptic children and in 24 age and sex-matched controls. The mean serum free carnitine level was significantly lower in patients taking valproic acid (33.5 +/- 13.1 nmol/ml) than in control subjects (50.8 +/- 14.6 nmol/ml) (p < 0.001). The mean blood ammonia level was significantly higher in patients receiving valproic acid (93.9 +/- 20.5 mu g/dl) than in controls (79.6 +/- 21.4 mu g/dl) (p < 0.002). The mean serum beta-hydroxybutyrate level was significantly lower in patients taking valproic acid (2.26 +/- 2.24 mg/dl) than in controls (4.38 +/- 2.43 mg/dl) (p < 0.001).