1. A gas chromatographic method was applied to study plasma levels of perphenazine (PPZ) and its major metabolites in man before and during simultaneous antiparkinson treatment. Twenty-six psychotic patients received various forms of PPZ administration as well as antiparkinson drugs. 2. Biperidine (5 mg) was administered intravenously to each of five patients, who 5 days earlier had had a single dose of PPZ-enanthate i.m. No significant alterations in plasma concentrations of PPZ were observed. 3. In fourteen patients receiving oral PPZ treatment the plasma levels of PPZ and its metabolites did not deviate significantly from controls after addition of biperidine or orphenadine given for 3 weeks in fixed oral doses. 4. The ratio between PPZ plasma concentration measured 4 and 7 h after the morning dose was not affected by concomitant antiparkinson therapy. 5. It is concluded that no clinically relevant pharmacokinetic interaction takes place between PPZ and two generally used antiparkinson drugs during steady-state conditions in psychotic patients.