Plasma GH responses to GHRH and somatostatin were examined in 43 patients with active acromegaly. Thirty-two of these patients showed GH increases 50% above the basal values in response to at least 1 of 3 stimuli (TRH, LHRH, arginine) (categorized as group I). The remaining 11 patients showed no response to any of the stimuli (categorized as group II). Following somatostatin infusion, group I (n = 31) showed significantly greater GH suppression than group II (n =11) from 30 to 90 min (p<0.05-0.01). In addition, plasma GH responses to GHRH at 15 and 30 min was also greater in group I (n =12) than in group II (n = 5) (p <0.05 & 0.01). There was a positive correlation between the log value of the peak GH after GHRH and the maximal % decrement after somatostatin (r= 0.64, p<0.02). However there were no differences in the responses of normal thyrotrophs (TSH) to TRH between the two groups. These results indicate that there are two types of acromegaly, i.e., one is more responsive and another is less responsive to either non-specific (TRH & LHRH) or specific GH stimulations (GHRH & somatostatin).