To evaluate renal tubular function in children receiving antiepileptic drugs the urinary activity of two lysosomal enzymes, N-acetyl-beta-glucosaminidase and beta-galactosidase, were measured. The enzyme levels were determined before the administration of antiepileptic drugs and 8 months after. Fourteen epileptic children received valproate, and 17 received carbamazepine. The urinary activity of these enzymes in 25 healthy control patients also was examined. Increased N-acetyl-beta-glucosaminidase activity was found in 50% of patients taking valproate and in 17.6% of patients taking carbamazepine. Increased beta-galactosidase activity was found in 28.5% of patients taking valproate and 11.7% of patients taking carbamazepine compared with the results before treatment. On the basis of these results, it is suggested that patients taking antiepileptic drugs, especially valproate, may demonstrate minor signs of tubular dysfunction. In those patients who use these drugs at increased dosage levels or for long periods, the possibility of tubular dysfunction may be increased, and these dysfunctions may manifest in clinical symptoms.