Serum GH levels were measured in 14 acromegalic patients in ITT, TRH test, LH-RH test, arginine test, OGTT, exercise test, CPZ test and PEG. More than 50% increase of serum GH level was observed in 3 of 13 ITT, 8 of 14 TRH test, 6 of 14 LH-RH test, 2 of 12 arginine test, 2 of 9 OGTT and 2 of 7 PEG. Only 2 out of 7 cases showed more than 50% decrease of serum GH level in CPZ test. Striking GH responses were observed in TRH and LH-RH test. Five of 14 cases showed more than 100% increase of serum GH level in TRH test but they did not have a significant response to LH-RH. Four of 14 cases also had more than 100% increase in LH-RH test with no significant response to TRH. The rest of the cases (5 cases) did not have marked response to either TRH or LH-RH. According to these results, serum GH responses of acromegalic patients could be devided into 30 types, 1) TRH responding type, 2) LH-RH responding type, 3) no response type.