Rat pups, when socially isolated, emit ultrasonic vocalizations which are believed to indicate distress. This study investigated the effect of intracerebroventricular (ICV) administration of corticotropin releasing factor (CRF) on the production of ultrasonic isolation calls. Following a 2-minute baseline isolation test, rat pups (5-6 days old) were injected ICV with CRF or the CRF antagonist, alpha-helical CRF (9-41). Thirty minutes later, calls were significantly decreased following CRF (0.1 and 0.01 micrograms) and increased following the CRF antagonist (1.0 micrograms). These effects were not explained by changes in locomotor activity, thermoregulation, or plasma glucocorticoid levels following peptide administration. Peripheral administration of CRF (1.0 and 10.0 micrograms) did not alter the number of isolation calls.