The pharmacokinetics of phenylethylmalonamide (PEMA), one of the major metabolites of the antiepileptic drug primidone, have been studied in 6 healthy volunteers after administration of single 500mg intravenous and oral doses. Following intravenous administration, after a very short distributive phase (t1/2= 0.23-0.53h), the decline of the log-PEMA concentration with respect to time appeared linear. The pharmacokinetic parameters, calculated according to a 1-compartment open model, showed the following values (mean ± SD): terminal half-life, 15.7 ± 3.4h; apparent volume of distribution, 0.69 ± 0.10 L/kg; total serum clearance, 31.3 ± 6.6 ml/h/kg.After oral administration, peak serum concentrations occurred at 0.5 to 4 hours and the oral bioavailability was 86.4 to 95.9%.