The effect of experimental hyperprolactinemia and hypoprolactinemia on human chorionic gonadotropin (hCG) concentration was studied in 34 healthy volunteers between weeks 6 and 11 of normal pregnancy. Hyperprolactinemia was induced in 12 women with 150 mg of sulpiride daily for 2 weeks; treatment led to a mean plasma prolactin (PRL) rise from 14.6 to 84.1 µg/liter after 1 week and to 83.0 µg/liter after 2 weeks. Hypoprolactinemia was effected in 11 women with 5.0 to 7.5 mg of bromocriptine, daily for 2 weeks; treatment decreased the mean plasma prolactin from 19.2 to 6.0 µg/liter after 1 week and to 5.2 µg/liter after 2 weeks. These PRL changes were significant (P< .001) in comparison with the plasma PRL concentrations in the 11 control women. Hyperprolactinemia induced by the sulpiride treatment was accompanied by a significant decrease in hCG concentration, whereas no difference in hCG was seen between hypoprolactinemia and control groups. This may suggest a synergistic action of hCG and PRL, or a direct effect of sulpiride on hCG, but the final mechanism and biologic importance of the phenomenon remain to be investigated.