Serum concentrations of zuclopenthixol were determined in a group of 20 patients treated with a depot preparation, zuclopenthixol decanoate in Viscoleo. Clinical assessments according to a Clinical Global Impression (CGI) scale, Comprehensive Psychological Rating Scale (CPRS), 16-item subscale for schizophrenia, and the UKU side effect scale were performed on 3 consecutive days of injection. The serum concentrations showed a limited individual variation and a high and significant correlation between dose and serum concentration. One patient had a particularly high serum concentration of zuclopenthixol. This patient also had an elevated concentration of the N-dealkyl metabolite, but a low concentration of the sulphoxide. For serum concentrations versus clinical state and side effects some significant correlations were found. All correlations were positive, which means that the higher the serum concentration the poorer the clinical state of the patient. We think that this probably reflects a common clinical pattern of increasing the dose, when the antipsychotic response is unsatisfactory. The study also showed that for moderately ill patients, who were given the optimum dose of drug, the subgroup of patients not experiencing side effects had significantly lower serum concentrations than the subgroup with side effects.