The effects of an expanding supratentorial mass on neural function and local cerebral blood flow (LCBF) in the brainstem were investigated in 32 cats. A balloon was inserted into the supra tentorial epidural space and was inflated at a constant rate of 0.0197 ml⁄minute. The auditory brainstem response (ABR) and short-latency somatosensory evoked response (SSER) were measured for evaluation of electrical conduction through the auditory and lemniscal pathways, respectively (n=18) . The blink

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... x was monitored to investigate neural function of the brainstem reticular formation (n=8). LCBF was measured by the hydrogen clearance method in four regions, including the inferior colliculus (IC), medial lemniscus (ML), pontine reticular for mation (PRF), and reticular formation of the medulla oblongata (MORF). When ICP was raised to 40 to 60 mmHg, blood flow in the IC and PRF was markedly decreased relative to that in the ML and MORF. The R2 blink reflex disappeared at 40 to 70 mmHg of ICP, prior to the disappearance of ABR wave V, SSER II components, and the R1 blink reflex. Upon the appearance of anisocoria, the ABR wave V disappeared in all cases, while the SSER II components were preserved in 4 of 18 cases. When both ABR wave V and SSER II components had nearly disappeared, LCBF in the IC and ML decreased to less than 50% of control values. The balloon was deflated within 15 minutes of the appearance of anisocoria. One hour after the deflation, the R2 blink reflex remained lost in all cases. SSER II components and ABR wave V recovered in 10 of 14 cases and 5 of 18 cases, respec tively. Brainstem LCBF improved in the majority of cases in which neural function of the corre sponding region recovered. These results suggest that blood flow and neural function are more vulnerable to intracranial hypertension in the PRF and IC than in the ML and, if there is evidence of tentorial herniation, do not recover well following decompression.