The influences of diazepam and naloxone on the increase of plasma corticosterone level induced by morphine, pentazocine, ACTH, or picrotoxin were investigated in male mice. The results obtained were summarized as follows: 1) The increase induced by morphine or pentazocine of plasma corticosterone level was not affected by naloxone pretreatment but markedly suppressed by diazepam pretreatment. 2) The increase induced by ACTH of plasma corticosterone level was not affected by diazepam or naloxone pretreatment. 3) The picrotoxin markedly increased plasma corticosterone level, and the inceease was not affected by diazepam or naloxone pretreatment. This above results suggest that the increase induced by opioids of plasma corticosterone level seems to be rather related with other than opiate- or GABArerecptor.