Part I. In order to determine the effect of dose size on the bioavailability of phenytoin (Dilantinᴿ), a single intravenous dose of 15 mg/kg, single oral doses of 400, 800, 1600 mg, and 1600 mg in divided doses (i.e. 400 mg every three hours) were administered to six healthy male subjects. Values of V(max) and K(m) obtained from the intravenous dose were used to determine the extent of absorption from the oral doses. Although no statistically significant difference in extent of phenytoin absorption was observed, the time to reach maximum phenytoin serum concentrations increased significantly from 8.4 hours for the 400 mg dose and 13.2 hours for the 800 mg dose to 31.5 hours for the 1600 mg dose. Peak serum concentrations of 3.9, 5.7, 10.7, and 15.3 mg/1 were observed after the 400, 800, 1600 and 1600 mg divided doses, respectively. It is suggested that the prolonged, but complete, absorption of large phenytoin doses is due to a slow dissolution and continued absorption from the colon. Owing to the prolonged absorption of phenytoin, it may be necessary to use a larger oral than intravenous loading dose to produce similar maximum phenytoin serum concentrations. Part II. The effects of age and obesity on the pharmacokinetics of thiopental were studied in 7 morbidly obese (aged 25 to 46 years) and 22 lean patients (aged 25 to 83 years), who were primarily undergoing abdominal surgery. In all 29 patients, serum thiopental concentrations were determined by gas-liquid chromatography using a nitrogen-selective detector. Based upon total (bound+free) thiopental concentrations, the average (±S.E.) volumes of distribution (Vᵦ and V(ss)) were significantly larger in the obese (7.94 ± 1.72 1/kg and 4.72 ± 1.03 1/kg, respectively) than in the age-matched lean patients (1.95 ± 0.22 1/kg and 1.40 ± 0.16 1/kg, respectively). Clearance based on total thiopental concentrations normalized to total body weight (TBW) was not significantly different between the obese (0.18 ± 0.03 1/hr/kg) and lean patients (0.21 ± 0.02 1/hr/kg). However, total body clearance not normalized to TBW was significantly larger in the obese (24.98 ± 5.62 1/hr) than in the lean patients (11.86 ± 1.29 1/hr). The half-life of thiopental was significantly larger in the obese (31.87 ± 4.53 hours) than in the lean patients (6.61 ± 0.52 hours) and was primarily a function of the larger apparent volume of distribution for thiopental. The unbound fraction of thiopental in serum (range, 17.8% to 27.6%) did not depend on the degree of obesity, but was found to be greater with advancing age. The apparent volumes of distribution, Vᵦ and V(ss), were also related to age. No significant relationship was found between total body clearance with increasing age. Thus, the half-life of thiopental was positively correlated with age, and as in the obese study, was found to be primarily influenced by the apparent volume of distribution.