It has been suggested that the septum plays an inhibitory role in the behavioral function. Recent work has shown that the septum is heterogeneous from the neuroanatomical perspective. The present study examined the water intake of rats lesioned with kainic acid (0.5 microg/0.5 microl/site) on three septal subregions: anterior medial (MSa), posterior medial (MSp), and lateral (LS) sites. Drinking volume was enhanced mostly in rats with the MSp lesion, and so was locomotor activity. However, these two measures were not significantly correlated. This polydipsia induced by MSp lesion was also found in a chronic domain. Another experiment further determined the dipsogenic effects of polyethylene glycol (PEG; 20%) and hypertonic saline (1 M NaCl) in MSp lesioned rats. Water intake increased significantly after administration of the hypertonic saline treatment, but not after injection of PEG. However, this disparity approached a nonsignificant level 8 hr after thirst challenges were conducted. In addition to revealing septal hyperdipsia. these data suggest that the septal subareas can be functionally heterogeneous in drinking behavior. The dipsogenic response profiles for the cellular and extracellular thirst challenges could be differentially affected by kainate lesion in the MSp.