The time-courses of plasma carbamazepine concentrations were followed in six apparently healthy adult subjects who, at different times, took single oral drug doses of 200, 400, 500, 600, 700, 800 and 900 mg. There were some suggestions of impaired bioavailability of the drug when given in tablet form. The following values were obtained for various pharmacokinetic parameters: kabs = 0.176 +/- 0.209 h-1; k = 0.0203 +/- 0.0055 h-1; T1/2 = 37.5 +/- 13.1 h; VD = 0.825 +/- 0.1041 . KG-1; Clearance = 0.0163 +/- 0.0061 1 . kg-1. The elimination rate constant showed a statistically significant increase with increasing drug dose. This may help explain the clinical observation that the rate of rise of steady state plasma carbamazepine concentrations tends to decrease with dose increase in patients taking carbamazepine alone.