Cytosolic free calcium during focal cerebral ischemia and the effects of nimodipine on calcium and histologic damage. The role of calcium as a mediator in neuronal death during ischemia is now quite strong. Evidence supporting this link includes studies in cell cultures and measurements of calcium accumulation in the mitochondria during ischemia, as well as direct measurements of shifts in extracellular calcium using microelectrodes. Since it is dangerously high concentrations of the intracellular free calcium that have been hypothesized to lead to neuronal damage, direct in vivo measurements of this parameter in ischemia are important. A technique for the measurement of intracellular free calcium is described, along with data from studies that dramatically demonstrate the time course of changes in intracellular free calcium induced by focal ischemia. Additional data are also presented that indicate that cellular damage can be attenuated by the use of agents that block calcium channels (nimodipine, which blocks voltage-sensitive calcium channels, and MK-801, which blocks receptor-operated channels) and support the concept that these agents owe their beneficial effects to their ability to reduce the accumulation of intracellular calcium. 