We compared the adrenocortical responses to acute stress in two free-living populations of bush warblers, Cettia diphone by measuring the increase in circulating levels of corticosterone in response to capture, handling and restraint over one hour. We wished to test the hypothesis that populations living in more severe habitats with shorter breeding seasons are likely to suppress the adrenocortical response to acute stresses during the nesting phase. We chose the bush warbler because males show no parental care which also has been shown to be an ecological correlate of stress modulation. Male bush warblers responded to playback of songs and long calls with aggressive behavior that was identical at the two sites (Furano, Hokkaido and Chichibu, Honshu). Plasma levels of testosterone were also similar in the two populations suggesting that they were in the same reproductive state when tested. Males of both populations showed marked increases in circulating corticosterone levels during the capture stress protocol. We assumed that at the northern site, Furano, bush warblers would have a reduced adrenocortical response to stress compared with Chichibu (the southern site). Males at Furano, however, showed higher initial plasma levels of corticosterone and showed a greater response compared with males at Chichibu. Relatively less individual variation in the adrenocortical response to stress was seen at Chichibu compared with Furano, and less than in other passerine species studied to date. There was no relationship of body mass or fat score to initial corticosterone level, maximum level generated over one hour of capture stress, or the rate and percent increases of corticosterone. These data indicate that stress modulation may not be a simple consequence of shorter breeding season and more severe climate. Other ecological bases, as yet unknown, are involved.