In order to verify whether synthetic peptide histidine methionine (PHM-27) is able to induce serum prolactin (PRL) rise in normal subjects and to investigate its effect on PRL secretion in hyperprolactinemic conditions, PHM-27 (100 μg i.v. over 60 min at a rate of 3.3 μg/min) was given to 6 normal subjects and 11 hyperprolactinemic women, and serum PRL levels were measured before and at intervals up to 120 min after beginning the infusion. On a separate occasion, a saline infusion was administered to all these subjects as a control test. In normal subjects, PHM-27 caused PRL to increase from 7.8 ± 1.4 μg/l (mean ± SE) to 13 ± 2.1 μg/l (p < 0.05), the peak occurring at 30 min, whereas it did not significantly modify serum PRL levels in 11 patients with prolactin-secreting adenomas. We also observed a significant difference of serum PRL pattern during PHM infusion when compared to saline in normal subjects. In contrast, in hyperprolactinemic states, PRL curves were similar in both tests. Continuous infusion of PHM-27 was very well tolerated and caused no important adverse events. These data suggest that PHM, like vasoactive intestinal peptide, can participate in the regulation of PRL release under physiological conditions and that the unsignificant PRL increase in PRL-secreting tumors may reflect abnormalities of the PRL secretion mechanism in these pathological states.