Numerous reports have suggested that anoxic depolarization is a critical event in the pathogenesis of cerebral ischemia. Extracellular glutamate concentration ([Glu]e) is closely related to the pathogenesis of ischemia. Therefore, these pathogenic mechanisms merit study, especially the relationship between [Glu]e elevation and the ionic basis of early changes in membrane potential after ischemic insult in vivo. It is often presumed from electrophysiological studies that a causal relationship exists between impaired glutamate uptake and/or progressive glutamate increase and anoxic depolarization, but few in vivo reports have found any sign of a progressive increase of [Glu]e elevation preceding anoxic depolarization. Recently, we reported the application of an oxygen-independent real-time technique for monitoring glutamate levels in the extracellular space during in vivo ischemia, and demonstrated that the massive glutamate release during ischemia is biphasic. In the present study, using this real-time monitoring system, we carried out a more detailed analysis of the initial events in the first phase of glutamate release during ischemia-induced anoxic depolarization. The shape of the rising slope that forms the peak of the first phase suggested two components. The second component was approximately 10 times steeper than the first, with two different components of the rise on the way to the peak of the biphasic [Glu]e elevation. This is the first report to demonstrate these components of the initial glutamate peak, and suggests a progressive second component of the [Glu]e increase preceding Ca2+-dependent release from synaptic vesicles with anoxic depolarization, in vivo.