Monitoring multiple neurons is essential for understanding neuronal network activities. While calcium imaging from a population of cells is an effective method to study the network dynamics of a neural structure, it has been difficult to image from densely packed structures, such as the granule cell layer of the dentate gyrus, due to overlap of the cells. We have developed a novel method to label multiple granule cells with a Ca2+ indicator in rat hippocampal slices using Oregon Green 488 BAPTA-1 (OGB-1) AM. Synchronized burst activities (0.3–1.4 Hz), which were induced by applying 50 µm 4-aminopyridine, were monitored extracellularly with a glass electrode placed at the granule cell layer in the dentate gyrus. During the burst activities, spontaneously occurring action potential-induced Ca2+ transients from multiple (4–12) granule cells were monitored with a cooled CCD camera with single-cell resolution. Temporal structures of firing patterns from the multiple neurons were determined from Ca2+ transients. In each single-burst-event recorded from the extracellular electrode, each neuron fired synchronously within a 200 ms time window. The latency and its variance from the onset time of the single-burst-events to one of the Ca2+ transients decreased over time (< 7.5 min). These results indicate that the synchrony of the action potentials within a single-burst-event was enhanced as the burst activities proceeded. This progressive synchronization may be a key feature in making self-organizing neuronal networks.