A combination of K+/DC surface electrode and a fiberoptic fluorometric probe are applied to measurements in brain during cerebral ischemia. The kinetics of the responses of extracellular K+ activity and intracellular NADH fluorescence in the gerbil cerebrum following reversible carotid ligation are measured. K+e shows a two-phase response to carotid occlusion and an extended recovery phase following recirculation. The length of the recovery phase is dependent on the duration and severity of the ischemic period. In the gerbil model the degree of communication in the anterior circulation is variable, whereas a bilateral carotid occlusion is presumed to give complete cerebral ischemia. Pyridine nucleotide fluorescence serves as an indicator of the degree of ischemia. Bilateral carotid occlusions of up to 35 minutes in duration were performed. K+e reaches 30--50 mEq/liter in the extracellular space within the first two minutes. This represents cell depolarization and equilibration of K+ activity levels. Recovery appears to be complete in terms of the ability of the system to clear raised levels of K+e from the extracellular space.