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Previous work by the authors has established decreased brain ATP concentration after a combined hypoxic-hypotensive episode. This study was undertaken to determine what changes, if any, occur in brain norepinephrine (NE), dopamine (DA) and serotonin (5HT) concentration in association with hypoxia and hypotension, and to correlate any observed changes with simultaneously measured ATP concentrations. Rats were subjected to a 30 minute period of hypoxia (FI02 = .075) and hemorrhagic hypotension (MAP - 30 mm Hg) and then resuscitated. Significant increases (p .05) in cortical 5HT and DA concentrations were observed, at a time when ATP concentration was significantly (p .005) reduced. Additional experiments were conducted on rats depleted of 5HT by prior treatement with 5,7-dihydroxytryptamine. Equal decreases in ATP concentration were measured, and the cardio-vascular response to hypoxic-hypotension in 5HT depleted rats was similar to 5HT intact rats. We conclude that the increased 5HT after hypoxic-hypotension does not cause the decreased ATP concentration, nor does 5HT play a major role in cardio-vascular homeostasis under the conditions of this experiment. (Author)