Stroke occurs in all age groups, ranging from the new-born to the elderly. Our current understanding of the mechanisms of ischemic brain injury suggests that, despite age, the underlying cascade of events includes the rapid depletion of energy reserves, lactate accumulation, release of excitatory amino acids, high intracellular concentrations of Ca2+, and the production of oxygen free radicals. The extent to which these events affect brain injury, however, is profoundly influenced by age. Hyperglycemia for example, markedly enhances hypoxic-ischemic brain damage in adults, but has a protective effect in new-born rats. Insulin-induced hypoglycemia, on the other hand, protects the adult brain, but may be detrimental to the new-born. Substrate utilization of ketone bodies is markedly enhanced in the new-born, and has now been shown also to protect the brain. The immature brain is generally believed to be more resistant to the damaging effects of cerebrovascular compromise compared to the more mature brain. However, recent experiments suggest that the correlation between brain damage and age is not linear. To further clarify the effects of age and development on hypoxic-ischemic brain damage, we developed a model whereby rats of increasing age received identical cerebrovascular insults. Neuropathologic assessment at 7 days of recovery showed that brain damage was most severe in the 1- and 3-week-old animals followed by those that were 6 months. The 6- and 9-week-old groups had significantly less injury than the other three age groups. Hippocampal damage was most severe in the 3-week and 6-month-old rats compared to all other age groups. These findings contrast previously held beliefs regarding the enhanced tolerance of the immature brain to hypoxic-ischemic damage and demonstrate that the immature brain is, in fact, less resistant to hypoxic-ischemic brain damage than its adult counterpart. The results emphasize the need for a greater understanding of the effects of ontogeny on hypoxic-ischemic brain damage, particularly as it pertains to the development of therapeutic interventions.