The possible involvement of endogenous corticotropin-releasing factor (CRF) in the anxiogenic and pituitary-adrenal-axis-activating effects of cholecystokinin octapeptide sulfate ester (CCK 8) was investigated in rats. Intracerebroventricularly (i.c.v.) administered CCK 8 induced an anxiogenic response in an elevated plus-maze test, and enhanced the plasma corticosterone level. Pretreatment with different dilutions (1:10, 1:20 and 1:100, i.c.v.) of CRF antiserum and different doses of a CRF receptor antagonist, alpha-helical CRF (ahCRF, 0.001-1.0 microgram, i.c.v.) prevented the anxiogenic response to CCK 8 in a dose-dependent manner. None of the doses of CRF antiserum or ahCRF alone produced any alteration in either the elevated plus-maze paradigm or corticosterone level in saline-treated control rats. The results strongly suggest that the anxiogenic and hypothalamo-pituitary-adrenal-activating effects of CCK 8 are mediated via CRF.