Recent observations concerning the limits of tolerance of the brain to anoxia and ischemia and current speculation over the role of oxygen free radicals in the evolution of irreversible neuronal injury have focused increased attention upon the regulation of mitochondrial oxidative phosphorylation in general, and the effects of these insults upon cerebral mitochondrial behavior in particular. Among available techniques, only cortical isosbestic microfluorometry permits continuous, non-destructive analysis of the cerebral mitochondrial redox state during metabolic perturbations. We propose to employ isosbestic fluorometry in studies of canine cerebral cortex designed to confirm (or refute) the quantitative nature of the technique; to examine in detail a remarkable phenomenon observed in preliminary studies of cerebral anoxia, i.e. transient oxidation of NADH; to employ selected metabolic inhibitors and uncouplers to elucidate mechanisms whereby mitochondrial respiration is regulated during anoxia, graded hypoxia, and ischemia; and to evaluate, where appropriate, metabolic interventions (e.g. barbiturate therapy) purported to protect the brain during anoxia and ischemia with respect to their effects upon mitochondrial respiration.