The effect of corticotropin-releasing factor (CRF) on ultrasonic vocalizations (USVs) and other behaviors of isolated rat pups was examined at 5/6-, 9/10-, and 13/14-days. The hypothesis tested was that central CRF affects USV rate biphasically: as endogenous CRF increases from low basal levels it initiates USV production, but at higher levels CRF diminishes USV production. As predicted, the largest doses of CRF (0.1 and 1 μg) and of its antagonist alpha-helical CRF9−41 (aH-CRF; 20 μg) administered intracerebroventricularly (ICV) reduced USV rate compared to saline treatment during a 2-min isolation at ambient temperature in pups of all ages. Other behaviors were either unaffected or increased by drug treatment. Effects were not attributable to sedation or to a change in core temperature. Peripheral administration of 1 μg CRF or 20 μg aH-CRF had no effect. When isolation occurred in a heated chamber containing soiled bedding from the home cage to minimize baseline USV rate, ICV-CRF (0.001, 0.01, 0.1, or 1 μg) did not enhance the rate. The quieting ability of aH-CRF is evidence that central endogenous CRF enhances the rate of ongoing USVs during brief isolation; the quieting ability of CRF suggests that this peptide may also be responsible for the reduction in USV rate that normally occurs during more protracted isolation. However, CRF alone is not sufficient to induce vocalizing in quiet pups.