The purpose of this study was to investigate the prolactin (PRL)-releasing mechanism of the hypothalamo-pituitary axis in pregnancy. Forty-six gravidas in the first and second trimesters received bromocriptine (BRC), 2.5 mg orally, metoclopramide (MCP), 10 mg intravenously, or thyrotropin-releasing hormone (TRH), 500 micrograms intravenously. Additionally, BRC was given orally to another 42 gravidas 60 minutes prior to the intravenous injection of MCP or TRH. The plasma PRL levels decreased significantly after BRC and remained significantly elevated after MCP or TRH administration. However, there were no significant differences of PRL response to these agents between the first- and second-trimester groups. The PRL release from the pituitary by MCP or TRH was suppressed significantly by pretreatment of BRC in the gravidas. We concluded that the control mechanism of PRL secretion remained unchanged in the first and second trimesters.