Thirteen acute psychotic patients received continuous oral treatment with perphenazine for a period of 8 weeks. Blood samples for quantification of perphenazine, perphenazine sulphoxide and prolactin were drawn twice weekly. Simultaneously, the therapeutic outcome and the degree of extrapyramidal side effects were recorded. The following conclusions were made: 1. In accordance with results achieved in one of our earlier investigations, a high risk of provoking extrapyramidal side effects was associated with plasma levels of perphenazine exceeding about 3 nmol/l. 2. An excellent therapeutic outcome was associated with plasma concentrations of perphenazine above 1.5 nmol/l. 3. Plasma concentrations of perphenazine and prolactin were poorly correlated to each other (R = 0.49). 4. A significant correlation (R = 0.85) was shown between the scores for the Brief Psychiatric Rating Scale and the Comprehensive Psychopathological Rating Scale.