The recently synthesized depot neuroleptic haloperidol decanoate (HD) has been mainly used in the treatment of chronic schizophrenia. In this trial 23 outpatients, 11 male and 12 female, suffering from schizophreniform disorders, according to the DSM III, were treated with 50 mg to 250 mg of HD, every 4 weeks for 12 months. At the end of the trial, the mean dosage of HD administered and BPRS (Brief Psychiatric Rating Scale) mean scores were significantly lower than the initial ones (70.8 mg VS 108 mg, and 21.5 VS 45.7 respectively). In 10 male patients, plasma levels of haloperidol and PRL were evaluated during the first four weeks of treatment. A positive correlation between haloperidol levels and clinical improvement and a significant increase in PRL levels in correspondence with haloperidol plasma peaks, but not with clinical improvement, were observed. These data indicate a dissociation between the effect of haloperidol after HD injection on the limbic dopaminergic system and the effect at the level of the tubero-infundibular system.