Two different single doses (400 and 600 micrograms) of the new long-acting dopamine agonist cabergoline (CBG) were given to 12 normal cycling women, 17 puerperal women, and 24 hyperprolactinemic women (12 with idiopathic hyperprolactinemia and 12 with pituitary adenoma). Plasma PRL was determined in blood samples collected before and at frequent intervals for 5 days after CBG administration. Both CBG doses induced marked inhibition of PRL secretion in all women. A decrease in plasma PRL levels was evident 1-2 h after CBG administration and persisted for up to 5 days. The 600-micrograms CBG dose had a more potent (P less than 0.05) PRL inhibitory effect than the 400-micrograms dose in normal, puerperal, and hyperprolactinemic women. Moreover, while 400 micrograms CBG prevented lactation in 3 of 7 puerperal women, 600 micrograms CBG prevented lactation in 5 of 5 puerperal women. A moderate blood pressure decrease occurred 3-6 h after CBG treatment, but no other side-effects occurred. These results demonstrate that CBG induces a dose-related inhibition of PRL secretion in normal women as well as in puerperal and hyperprolactinemic women. The potent long-lasting PRL inhibitory effect of CBG in conjunction with the absence of side-effects typical of dopaminergic compounds suggest that this drug is an advance in the medical treatment of hyperprolactinemia.