Electrical and chemical synapses provide two distinct modes of direct communication between neurons, and the embryonic development of the two is typically not simultaneous. Instead, in both vertebrates and invertebrates, gap junction-based electrical synapses arise before chemical synaptogenesis, and the early circuits composed of gap junction-based electrical synapses resemble those produced later by chemical synapses. This developmental sequence from electrical to chemical synapses has led to the hypothesis that, in developing neuronal circuits, electrical junctions are necessary forerunners of chemical synapses. Up to now, it has been difficult to test this hypothesis directly, but we can identify individual neurons in the leech nervous system from before the time when synapses are first forming, so we could test the hypothesis. Using RNA interference, we transiently reduced gap junction expression in individual identified neurons during the 2-4 d when chemical synapses normally form. We found that the expected chemical synapses failed to form on schedule, and they were still missing months later when the nervous system was fully mature. We conclude that the formation of gap junctions between leech neurons is a necessary step in the formation of chemical synaptic junctions, confirming the predicted relation between electrical synapses and chemical synaptogenesis.