The hypouricemic effect of chlorprothixene (Taractan), a major tranquilizer from the group of thioxanthenes, was evaluated in 30 psychiatric patients who took the drug as part of their regular treatment. Levels of serum uric acid, urea, and creatinine before, during, and after the treatment were measured, as well as creatinine clearance and uric acid clearance before and during the treatment. A uricosuric effect, resembling that of probenecid, was found that exerts itself in all the patients, regardless of age, sex, diagnosis, and associated drugs. The resulting hypouricemia starts as soon as 24 hours from the beginning of treatment, stabilizes within 10 days, and averages, at that time, 48% of the initial level. It is reversible within 10 days from the end of treatment.