Simultaneous optical measurements of extra- and intracellular Ca2+ concentrations were carried out on isolated snail neurons injected iontophoretically with Ca2+. The fluorescent indicator Fura-2 was used to measure intracellular concentration of free Ca, and the absorbant indicator Antipyrylazo III to measure changes in extracellular calcium concentration in the micro-chamber containing the cell. The velocity of Ca2+ extrusion from a single cell has been shown to be in accordance with the level of free Ca in the neuronal cytoplasm. After an increase in intracellular free Ca by iontophoretic injection from a microeletrode to 0.2-0.5 microM, the velocity of Ca2+ extrusion from the neuron was approximately 0.3-4.6 microM/sec per cell volume. During caffeine-induced calcium-dependent calcium release of Ca2+ from intracellular stores a stimulation of calcium extrusion took place, reaching the velocity of 5.0 microM/sec per cell volume.