The relationship between the serum carbamazepine levels and dosage is obscure. The present study dealt with the daily fluctuation and the dosedependency of serum carbamazepine levels in epileptic patients under a long-term treatment with carbamazepine. The blood was taken by venous puncture at intervals of three hours from 0O:OO to 2 1 : 00 from 17 inpatients, and also taken from 17 outpatients at 1O:OO and from 35 outpatients at 15:OO. The serum carbamazepine levels were determined by the enzymemultiplied immunoassay.The results obtained were as follows: 1. The mean daily fluctuation of serum carbamazepine levels was the lowest at 06:OO and the highest at 15:OO in 17 in-patients.2. The serum carbamazepine levels ( n = 52) at 15:OO showed a linear relationship to the oral dosage (p < 0.001 1, but the levels (n = 34) at 1O:OO did not.The levels in patients with generalized epilepsy were lower than those with partial epilepsy when the daily dosage increased over 10.05 mg/kg ( p < 0.05).These data suggested that the serum carbamazepine levels in the afternoon showed dosedependency, and that the therapeutic range should be determined by monitoring the serum carbamazepine levels in the morning which showed the lowest levels and the toxic range should be in the afternoon which showed the highest. 3.