There is evidence that valproic acid causes a reduction of serum biotinidase enzyme activity. We determined the serum concentration of antiepileptic drugs, transaminases, gamma-glutamyl transferase, ammonia, and biotinidase enzyme activity in 57 children treated with valproic acid, in 17 children treated with carbamazepine, and in 75 age- and sex-matched healthy controls. There were no significant differences in the serum biotinidase enzyme activity between the patients treated with valproic acid, the patients treated with carbamazepine, and the control group. Hyperammonemia was detected in 8 patients treated with valproic acid. Hair loss was observed in 3 female patients treated with valproic acid, and the alopecia disappeared with the oral administration of biotin (10 mg/ d) in 3 months. These results suggest that the treatment with valproic acid does not alter the serum biotinidase enzyme activity.