Six normal volunteers were tested to ascertain the dose-dependency and reproducibility of the prolactin response to metoclopramide. Furthermore, pretreatments with 1-dopa or cyproheptadine were performed. The same subjects also received intravenously 500 microgram TRH. The oral administration of 5 mg metoclopramide produced a significant rise in prolactin at 60 min and a maximal elevation at 90 min. The intravenous administration of 5 mg metoclopramide produced a significant increment at 15 min and a maximal prolactin level at 30 min. Peak prolactin levels following i.v. administration of the drug were greater than those after oral administration. Intravenous administration of 1 mg of metoclopramide produced a potential increment in plasma prolactin. Maximal prolactin response to 1 mg i.v. metoclopramide was similar to that found with 500 microgram i.v. TRH. Reproducibilities of prolactin response were good in all doses and modes of administration of metoclopramide. Pretreatment with 1-dopa inhibited the metoclopramide-induced prolactin response, but pretreatment with cyproheptadine did not influence the prolactin response to metoclopramide. Metoclopramide is a good and reproducible stimulator of prolactin secretion.