Seventeen outpatients were treated with depot neuroleptics, zuclopenthixol decanoate in Viscoleo or fluphenazine decanoate in sesame oil, with dosage intervals of 3 weeks. During the 4th, 6th, and 8th dosage interval blood samples were drawn in oxalated tubes. Plasma concentrations of the active neuroleptic drugs, zuclopenthixol and fluphenazine, were determined by high performance liquid chromatography. The concentrations indicated some interindividual as well as intraindividual variations. For zuclopenthixol the maximum concentration was most often seen at day 7 after injection, whereas the kinetics of the fluphenazine concentrations was more variable. There was an indication of more fluctuation in the 4th dosage interval than in the 8th dosage interval, possibly due to the fact that steady state has not yet been achieved at the 4th dosage interval.