Serum levels of phenobarbital, and also urinary excretion of phenobarbital and p-hydroxyphenobarbital, were examined after single and repeated oral doses of phenobarbital to three male subjects. Serum levels of phenobarbital at steady state were approximately ten times as high as those after a single dose. The overall elimination rate constant for loss of phenobarbital from serum, Kel, was significantly reduced after repeated doses, and Cmax infinity values calculated from single-dose data poorly predicted observed Cmax infinity values. Five-day urinary excretion of phenobarbital and p-hydroxyphenobarbital accounted for 16 and 21 per cent, respectively, of the initial dose. Due to extensive drug accumulation, 83 per cent of the final dose was excreted in five-day urine as phenobarbital and 85 per cent, as p-hydroxyphenobarbital. Comparison of plasma and renal clearances indicated that the rate of phenobarbital metabolism was reduced owing to repeated dosing, while the rate of urinary excretion of parent drug was unchanged.