The role of corticotropin-releasing factor (CRF) in four model stresses (cold, ether, immobilization, and trauma) was examined in the guinea pig by using passive immunoneutralization with anti-CRF antiserum. Plasma corticotropin levels were measured at various times after exposure to stress, and groups treated with CRF antiserum were compared with those treated with normal rabbit serum. Of the four stresses tested, ether had the most pronounced effect on corticotropin secretion. Treatment with anti-CRF inhibited most of the ether-induced corticotropin secretory response, the difference between the normal serum- and the anti-CRF antiserum-treated groups being significant at 5 and 10 min (P less than 0.01). Corticotropin responses to cold stress in the two groups differed at the 0.05 level of significance at 10 and 20 min. After administration of trauma (leg fracture), a statistically significant difference (P less than 0.01) between the two groups also was evident, albeit only at 20 min. During immobilization, corticotropin levels differed significantly from control only in the normal serum-treated group but not in the anti-CRF-treated group. These findings show that CRF antiserum was effective in reducing corticotropin levels, indicating that CRF has an important role in mediating corticotropin response to stress. The fact that neutralization was incomplete might be due to an inability of the antiserum to sufficiently neutralize the endogenous CRF or, more likely, reflects the contribution of additional mediators, notably catecholamines and vasopressin, of corticotropin release upon stress.