Fluctuations in the carbamazepine (CBZ) concentration during the day were studied using the profiles of 88 patients on slow release CBZ preparations. Blood was taken at 800, 1100,1400,1700, 2000, 2200 and 800 hr of the following day. The CBZ dosage was divided into two equal doses and administered at 800 and 2000 h. The influence of different factors on the fluctuations in the CBZ concentration during the day was studied. The fluctuation correlated negatively (r = −0.51, p < 0.001) with the level-dose ratio LDR (CBZ morning concentration-CBZ dose per body weight ratio). The co-medication and preparation had no additional significant influence on fluctuations in the CBZ concentration during the day. The maximal CBZ serum concentration during the day can be described for most patients as a function of the CBZ morning concentration and the level-dose ratio using a suitable regression equation (r = 0.93, standard error of estimate = 1.2 mg/L).