The role of the amygdala in the regulation of hypothalamic release of corticotropin-releasing factor (CRF) was investigated. Microinjection of glutamate (50 nmol) into the amygdala resulted in increased plasma corticosterone in male rats previously subjected to a 14-day unpredictable stressor paradigm (p < or = 0.05 vs. saline-injected controls). A long-lived increase in corticosterone levels was also observed in rats which were urethane-anesthetized (1.35 g/kg) 3 h prior to glutamate microinjection (p < or = 0.01 vs. saline-injected controls). These effects on plasma corticosterone were observed despite the presence of high basal levels of corticosterone. Furthermore, microperfusion of glutamate (3-300 microM) into the amygdala of urethane-anesthetized rats resulted in a dose-dependent increase in CRF release from the median eminence, as assessed by in vivo microdialysis (p < or = 0.025 vs. basal). These results indicate a facilitating role for the amygdala in stress-induced increases in CRF release and subsequent adrenocortical activation.