The presence of hyperglycemia before brain ischemia increases stroke-related morbidity and mortality in experimental animals and humans. However, little is known of the effect of hyperglycemia on regional cerebral blood flow (rCBF). Acute hyperglycemia was induced in awake but restrained rats by intraperitoneal injection of 50% D-glucose. Regional flow was determined using [14C]iodoantipyrine and quantitative autoradiography. Elevation of plasma glucose from 11 to 39 mM was associated with a 24% reduction in rCBF when compared with controls that received normal saline. Intraperitoneal D-mannitol produced an elevation of plasma osmolality equivalent to that observed with glucose. However, rCBF was only reduced by 10%. Hyperglycemia appears to produce a global decrease in rCBF in awake rats that cannot be completely explained by the attendant increase in plasma osmolality. If a similar influence is present during brain ischemia, hyperglycemia could extend areas of critical flow limitation.