The factors responsible for the unusual susceptibility of the hippocampus to seizures and ischemic cell damage are not well understood. The CA1 pyramidal subfield of the hippocampus is particularly vulnerable to seizure activity and damage after ischemia. The possibility was examined that regional differences exist in extracellular volume, which might influence neuronal excitability and response to injury in the hippocampus. CA1 stratum pyramidale exhibited an exceptionally low extracellular volume fraction (EVF) of 0.12, whereas the EVFs of CA3 and dentate were considerably higher--0.18 and 0.15, respectively. The EVF of CA1 stratum pyramidale was reversibly reduced by 30 percent when the extracellular potassium concentration was raised from 3.5 to 8.5 mM, a procedure that induced spontaneous electrographic seizures in CA1. Thus there are regional variations in the properties of the extracellular space in the hippocampus that might underlie the propensity of the CA1 region to develop seizures and to suffer damage after ischemia.