For the study of brain stem auditory evoked response (BAER) changes due to acutely expanding mass lesions, posterior fossa balloon catheters were inflated slowly in anesthetized cats while supratentorial and infratentorial intracranial pressure and vital signs were monitored. Reliable changes in BAER occurred before the Cushing response and were reversible by balloon deflation. Because BAER changes precede the agonal changes of the Cushing response, serial BAER recording in patients with known or suspected posterior fossa masses may be useful in the management of these lesions.