In 264 epileptics undergoing long-term therapy with anticonvulsants, significantly reduced plasma biotin levels were found compared with a normal control group: 74% of the epileptics had biotin levels for those treated with sodium valproate were higher than for those treated with phenytion, primidone, or carbamazepine. The observed reduction in biotin levels might be a factor influencing the efficacy of these three antionvulsants.