The study concerned 7 patients suffering from schizophrenic disorder according to the DSM III R criteria, treated with a stable dose of haloperidol decanoate (Haldol décanoas) added with fluoxetine (Prozac) from 20 mg to 40 mg/day because of major depression. Patients were assessed at baseline and weekly during the first cycle, and then once a month before each haloperidol decanoate injection, using the brief psychiatric rating scale (BPRS), the general clinical impression scale (CGI) and the Montgomery and Asberg depression rating scale (MADRS). Extrapyramidal and anticholinergic side-effects, blood pressure and pulse were noted. Determinations of plasma and red blood cells concentrations of haloperidol and reduced haloperidol, and of fluoxetine and norfluoxetine, were conducted at the same time than clinical evaluations. For all patients, we observed an improvement by the end of the first week, which became significant at the end of the second week, and continued in subsequent weeks (more than 30 per cent). Two weeks after the addition of fluoxetine, a very significant increase in haloperidol concentrations (more than 100 per cent) was noted; fluoxetine seems to have pharmacokinetic interactions with haloperidol, either by inhibiting its hepatic metabolism (inhibition of cytochrome P450 isoenzyme) or/and by displacing it from protein binding sites.