The constancy of plasma osmolality demands that salt and water concentration within the extracellular fluid be constantly monitored and regulated within a few percentage points. The circumventricular organs in general, and the subfornical organ in particular, have long been proposed to be the site of the osmosensitivity. Isolated subfornical organ neurons of male rats were studied using the whole-cell patch-clamp technique and both action potential frequency and whole cell currents were measured as bath osmolality was changed, from 240 to 330mOsm, by altering the amount of mannitol and maintaining the concentrations of electrolytes constant. Out of 64 cells, 66% responded to changes in bath osmolality in a predictable manner, exhibiting a hyperpolarization and decrease in spike frequency in hypo-osmotic solutions and a depolarization and increase in action potential frequency during hyperosmotic exposure. Cells (34%) defined as non-responders exhibited no significant modulation during identical changes in extracellular osmolality. The responses to changing extracellular osmolality were dose dependent; the activity of subfornical organ neurons was significantly modulated by changes in extracellular osmolality of less than 10mOsm. By regression analysis, this osmosensitivity was approximately 0. 1Hz/mOsm change throughout a +/-10mOsm range and was maintained throughout the range of osmolalities studied (270-330mOsm). The mechanism underlying this osmosensitivity remains unclear, although the non-selective cation conductance and the volume-activated chloride conductance do not seem to be involved.This intrinsic osmosensitivity of subfornical organ within the normal physiological range supports the view that this circumventricular structure plays a role in normal osmoregulation.