Spatial and temporal changes in the intracellular free Ca2+ concentration in response to Ca2+ influx at the cell membrane and to Ca2+ release from intracellular organelles were studied by recording fluorescence of Ca2+-sensitive probes, fura 2 or indo 1, with conventional epifluorescence or confocal laser-scanning microscopy combined with recordings of Ca2+-dependent membrane responses in bullfrog sympathetic ganglion cells. It was found that an increase in the intracellular Ca2+ induced by (an) action potential(s) in freshly isolated ganglion cells bathed in Ringer's solution was solely a result of Ca2+ influx, while a rise in the intracellular Ca2+ by Ca2+ current in voltage-clamped cultured neurones was caused by not only Ca2+ influx but also Ca2+ release. This Ca2+ release was suggested to occur by a voltage-dependent (and graded) mode of activation of a Ca2+-induced Ca2+ release mechanism, explaining the lack of Ca2+ release by action potentials (because of their short-lasting depolarization) in freshly isolated neurones. In both cases, there was an inward spread of an increase in intracellular Ca2+. On the other hand, all or nothing activation of Ca2+-induced Ca2+ release occurred in the presence of caffeine, leading to the oscillation of Ca2+ in the cells. Characteristics of this mode of Ca2+ release and unique properties of drugs to block Ca2+ release were described. Finally, the physiological significance of different types of Ca2+ release was discussed.Key words: Ca2+ current, Ca2+-induced Ca2+ release, Ca2+ transient, confocal microscope, bullfrog sympathetic ganglion cells.