In order to examine 5HT metabolism in the hypothalamo-pituitary (HP) system of chronic schizophrenic patients taking haloperidol for a short or long period, chronological changes of blood 5HT, PRL and HGH were measured after an oral loading dose of L-5hydroxytryptophan (L-5HTP), a precursor of 5HT. The subjects consisted of 8 male patients with chronic schizophrenia, who were divided into the following two groups. The 1st group--4 patients taking haloperidol (5.6 mg/day) for 8 months on an average (short-term treatment). The 2nd group--4 patients taking haloperidol (4.8 mg/day) for 7.5 years on an average (long-term treatment). The control group was made up of 9 healthy male volunteers. As a result, the basal level of blood 5HT in the 1st group was significantly higher than that in the control group. The blood 5HT levels in the 1st and 2nd groups showed an equally remarkable increase as compared with the control group. The basal level of plasma PRL in the 2nd group was significantly lowe than that in the control group. Moreover, in the 2nd group, an increase in the plasma PRL level after the loading was suppressed, but it showed less suppression in the 1st group. There was no significant difference in the basal HGH levels among the control, 1st and 2nd groups. After the loading, an increase in the plasma HGH was suppressed in the 2nd group, but the suppression was less in the 1st group.(ABSTRACT TRUNCATED AT 250 WORDS)