Positron emission tomography (PET) was used to investigate the regional hemodynamic and metabolic changes that accompany focal reductions in cerebral blood flow to ischemic but uninfarcted regions of the brain. Studies were performed on 7 patients chosen from a larger group of subjects with transient ischemic attacks and normal computed tomographic findings, specifically because their PET studies showed decreased blood flow to the symptomatic hemisphere rather than symmetrical flow to the two hemispheres. In regions with decreased blood flow, the cerebral metabolic rate for oxygen was also decreased, but to a lesser degree. Cerebral blood volume, vascular mean transit time, and fractional extraction of oxygen from blood were all increased. These findings indicate that cerebral oxygen metabolism was maintained in the face of decreased blood flow by local compensatory mechanisms that included dilation of intraparenchymal blood vessels and increased transfer of oxygen from blood to tissue. Such knowledge of the physiological characteristics of ischemic, uninfarcted brain is important if PET is to be used clinically to differentiate reversible cerebral ischemia from irreversible infarction.