The possible role of nitric oxide ( NO) in brain energy metabolism during perinatal asphyxia in the rat was studied. Exposure of early neonates to 5 min of anoxia significantly inhibited brain mitochondrial complex II-III activity by 25%, without affecting complex I, complex IV or citrate synthase activities. This insult was accompanied by ATP depletion (54%) and increased concentration of nitrites plus nitrates (1.4-fold), suggesting enhanced NO synthesis. Administration of N ω -nitro-l-arginine monomethyl ester (l-NAME) to the mothers inhibited neonatal brain NO synthase activity, as reflected by the decreased (23%) cyclic GMP concentration. These l-NAME-treated neonates showed complete resistance to anoxic-mediated brain mitochondrial complex II-III damage. Our results suggest that brain mitochondrial dysfunction leading to energy deficiency during perinatal asphyxia is a NO-mediated process.