Stroke in humans is usually associated with advanced age. Nevertheless, almost all animal models of ischemic stroke are based on young animals. The present study was designed to assess the effect of age on the development of ischemic injury in a model of focal brain ischemia in rats. Two age groups of Wistar rats were used: young adult (3 months) and old (24-26 months). Under halothane anesthesia, polyethylene microspheres (50 microm in diameter) were injected into the left common carotid artery following a temporary occlusion of the external carotid artery. Sham-operated rats underwent the same procedure but were injected with an identical volume (100 microl) of saline only. Rats of both experimental groups displayed neurological impairment after surgery. However, contrary to expectation, the young rats were more affected than the old rats. Young rats displayed an abrupt 30% decrement in neurological functions in the first week and then showed a partial functional recovery into a 12% decrement from the second week on. Old rats developed the neurological impairment gradually over a 2-week period (6.3% in the first week and 11% in the second week and thereafter). One month later, rats were tested in a water maze task. Again, performance was more impaired in the young ischemic rats than in the old rats. Histological evaluation revealed more extensive neurological damage in young ischemic as compared to old rats. Thus, although increased age has a critical effect on the evolution of the neurological impairment following focal brain ischemia and stroke, its effects in the rat model were more pronounced in the young animals.