Plasma mephobarbital and phenobarbital concentrations were determined in 11 epileptic patients receiving mephobarbital alone or in combination with other antiepileptic drugs. The analysis was carried out by a selective ion monitoring (SIM) mass fragmentography technique following formation of N-propyl derivatives of both drugs. The plasma concentrations of phenobarbital ranged from 4 to 32 μg/ml and those of mephobarbital from 0.2 to 1.7 μg/ml. Differences in the metabolism rates of the drugs accounted for the plasma concentration differences; mephobarbital is metabolized more rapidly than phenobarbital. Phenobarbital concentrations obtained by SIM mass fragmentography were similar to those obtained by gas-liquid chromatographic on-column methylation, which quantitates only “total phenobarbital” (mephobarbital plus phenobarbital).