The pharmacokinetics and bioavailability of phenobarbital were examined in six healthy adult subjects after a 2.6 mg/kg intravenous and a 2.9 mg/kg oral dose. Serum concentrations of phenobarbital were followed by means of a high pressure liquid chromatographic assay for 21 days after drug administration. After the intravenous dose, the mean distribution half-life was 0.18 hour and the mean elimination half-life was 5.8 days. Mean total body clearance and mean renal clearance were 3.0 ml/hr/kg and 0.8 ml/hr/kg, respectively. The apparent volume of distribution was 0.60 liter/kg. After administration of phenobarbital tablets, the maximum phenobarbital serum concentration was 5.5 mg/liter at 2.3 hours after the dose. Adjusted absolute availability of phenobarbital from the tablets studied was 94.9 per cent (range 81-111.9 per cent). The elimination half-life averaged 5.1 days for the oral dose. There was no evidence of autoinduction of phenobarbital elimination over the study period.