Permeability transition was examined in heart mitochondria isolated from neonate rats. We found that these mitochondria were more susceptible to Ca2+-induced membrane leakiness than mitochondria from adult rats. In K+ containing medium, at 25 °C, mitochondria were unable to accumulate Ca2+. Conversely, in Na+ containing medium, mitochondria accumulated effectively Ca2+. At 15 °C mitochondria accumulated Ca2+ regardless of the presence of K+. Kinetics of Ca2+ accumulation showed a similar Vmax as that of adult mitochondria. Lipid milieu of inner membrane contained more unsaturated fatty acids than adult mitochondria. Aconitase inhibition and high thiobarbituric acid-reactive substances (TBARS) indicate that oxidative stress caused mitochondrial damage. In addition, proteomics analysis showed that there is a considerable diminution of succinate dehydrogenase C and subunit 4 of cytochrome oxidase in neonate mitochondria. Our proposal is that dysfunction of the respiratory chain makes neonate mitochondria more susceptible to damage by oxidative stress.