Of 117 patients treated with anticonvulsants reduced plasma biotin levels were found in over 80% of cases. Significant negative correlations with the average daily dose and the total amount of anticonvulsants as well as with the plasma phenytoin level were also evident. It is proposed that the lowering of biotin--which is a co-factor of the carboxylating enzyme system--is an important mechanism in mode of action of anticonvulsants. Reduced biotin levels possibly lead to an accumulation of carbon dioxide or a diminution of aspartate in brain tissue. Low biotin levels may also be the cause of some well known side effects of anticonvulsants such as cerebellar disturbances and dermatitis. In comparison to the other epileptics the patients with cerebellar symptoms showed significantly lowered plasma biotin levels.