In a six-week randomized, double-blind study the efficacy and side effects of perphenazine decanoate (PD) and perphenazine enanthate (PE) were evaluated and compared in 26 and 24 acute psychotic patients respectively. Of either formulation 100 mg were administered intramuscularly every two weeks. Maximum and minimum plasma concentrations of perphenazine were measured for each injection period using gas liquid chromatography. There was no statistically significant difference between PD and PE in terms of overall antipsychotic efficacy, assessed by means of the Brief Psychiatric Rating Scale (BPRS). However, when an 'Amelioration Score' (AMS) of at least 50% of the totally obtainable scores was defined as individual response criterion it was revealed that the PD group only one patient (4%) did not meet this criterion, compared with six patients (25%) in the PE group. Extrapyramidal side effects were significantly more pronounced in the PE-treated patients, who also required significantly higher amounts of antiparkinson medication. The mean maximum concentration of perphenazine in plasma was 5.0 nmol/l in the PD, and 10.6 nmol/l in the PE-treated patients. The ratio of the mean maximum to the mean minimum concentration was 1.41 and 4.02 in the decanoate and enanthate groups respectively. In the patients treated with PD there were signs of accumulation indicating the possibility of prolonging dosage intervals. The present study yielded further support to previous findings demonstrating that intramuscular administration of PD dissolved in sesame oil, in contrast to PE, results in even and flat plasma perphenazine concentration curves, which not only provides a stable antipsychotic effect but also most likely carry the responsibility for the low incidence of extrapyramidal side effects observed.