CB-154 (2-Br-alpha-ergocriptine) stimulates growth hormone (GH) release in normal subjects. In acromegaly, however, this agent often decreases plasma GH level paradoxically. In order to examine the mechanism of the so-called "paradoxical decrease" in plasma GH with CB-154, GH responses to CB-154 were compared with GH responses to thyrotropin-releasing hormone (TRH), arginine, and luteinizing hormone-releasing hormone (LH-RH) in 20 cases of acromegaly. CB-154, as well as L-dopa, elicited decrease in GH in those patients whose GH secretion was more responsive to TRH and less responsive to arginine. These results suggest that, like L-dopa, CB-154 has similar dual actions of TRH antagonistic GH decrease and GH-RF (GH-releasing factor) facilitative GH increase. Moreover, it was speculated from this study that CB-154 has no significant effect on LH-RH release. The value of (increase ratio of GH on TRH)/(increase ratio of GH on arginine) can be used as an index for the indication of chronic CB-154 therapy in patients with acromegaly.