Perfusion of the forebrain with glucose at concentrations which alter neither plasma insulin nor glucose levels leads to sympathetic activation in some rats. We used the expression of Fos-like immunoreactivity (FLI) as an index of neuronal activation to examine the anatomic substrate underlying this phenomenon. Male Sprague-Dawley rats were infused via the right internal carotid artery with glucose (4 mg/kg/min) or equiosmolar mannitol for 60 min. They were killed 3 h after infusion onset and their brains reacted for FLI. As compared to mannitol-infused controls, 105% and 117% more neurons in hypothalamic ventromedial nucleus (VMN) and parvocellular portion of the paraventricular nuclei (PVN) of glucose-infused rats showed FLI, respectively. Importantly, only about half the glucose-infused rats showed increased FLI cells in these areas when compared to controls. In these same animals, glucose also significantly activated cells in the dorsomedial n. There was little FLI expressed in the magnocellular neurons of the PVN. This selective glucose response was bilateral in keeping with the bilateral distribution of India ink to midline hypothalamic structures following unilateral carotid infusions. Retrograde transport of cholera toxin B from medullary and thoracic spinal cord sympathetic outflow areas showed labeling of about 10% of PVN neurons with FLI activated by intracarotid glucose. There was no double labeling of VMN neurons. This supports the presence of anatomic pathways by which a subpopulation of glucose responsive PVN neurons might activate the sympathetic outflow areas in the medulla and spinal cord. The apparent bimodal distribution of glucose-induced activation of VMN and PVN neurons is in keeping with a similar bimodal pattern of sympathetic activation which obesity-prone but not obesity-resistant rats show following glucose infusions. Taken together, these data support a role for glucose-sensitive VMN and parvocellular PVN neurons in the weight gain phenotype specific sympathetic activation to glucose.