To determine the site of action of dopaminergic drugs on human PRL and GH release from pituitary adenomas, five PRL-and five GH-secreting adenomas were incubated with and without dopamine and L-dopa. Bromocriptine was also tested in order to compare its effect to that of the other drugs. In all of the experiments except one, a decrease of PRL, which was often statistically significant, was observed. When pooling the results of the PRL-secreting adenomas, the mean levels of PRL with dopamine, L-dopa, and bromocriptine were, respectively, 49%, 55%, and 60% of the control levels. In the GH-secreting adenomas, they were 60%, 67%, and 55% of that of the control. For GH, a decrease of the release was observed in four out of five GH-secreting adenomas. When pooling the results from these tumors, the mean levels of GH with dopamine, L-dopa, and bromocriptine were, respectively, 63%, 76%, and 64% of the control levels. In one case, a significant increase of GH was observed with the three dopaminergic drugs. This study produced the following conclusions. 1) Dopamine acts directly on PRL and GH release from human pituitary adenomas; in vitro, L-dopa effects are similar (its action probably occurs after conversion to catecholamines). These observations strongly suggest the presence of dopaminergic membrane receptors on human lactotroph and somatotroph adenomatous pituitary cells. 2) In vitro hormonal results are in good agreement with in vivo dynamic tests using L-dopa and bromocriptine. 3) The paradoxical effect of dopaminergic drugs on GH secretion in acromegalic patients may be attributed to modified dopamine membrane receptors. However, the paradoxical response is not a constant feature in acromegaly, and its mechanism needs further investigations.