Developing spinal networks undergo dramatic structural and/or functional rearrangements to achieve the maturation of motor control. Intracellular calcium changes are crucial signals in spinal network development, since transient elevations of intracellular Ca²⁺ might detail the emergence of specific phenotypes or guide the formation of cellular connectivity. Embryonic spinal neurons maintained in organotypic slice culture are known to mimic certain maturation-dependent signaling changes. Organoptypic spinal slices recapitulate during in vitro growth many molecular and physiological events of spinal networks formation in vivo, therefore they are ideally suited to investigate the dynamic of intracellular calcium signaling in clusters of neurons during their maturation. With such a model we investigated, in embryonic mouse spinal segments, the age-dependent spatio-temporal control of intracellular Ca²⁺ signaling generated by neuronal populations in ventral circuits and its relation with electrical activity. We used Ca²⁺ imaging to monitor areas located within the ventral spinal horn at 1 and 2 weeks of in vitro growth. Primitive patterns of spontaneous neuronal Ca²⁺ transients (detected at 1 week) were typically synchronous. Remarkably, such transients originated from widespread propagating waves that became organized into large scale rhythmic bursts. These activities were associated with the generation of synaptically-mediated inward currents under whole cell patch clamp. Such patterns disappeared during longer culture of spinal segments: at 2 weeks in culture, only a subset of ventral neurons displayed spontaneous, asynchronous and repetitive Ca²⁺ oscillations dissociated from background synaptic activity. We observed that the emergence of oscillations was a restricted phenomenon arising together with the transformation of ventral network electrophysiological bursting into asynchronous synaptic discharges. This change was accompanied by the appearance of discrete calbindin immunoreactivity against an unchanged background of calretinin positive cells. It is attractive to assume that periodic oscillations of Ca²⁺ confer a summative ability to these cells to shape the plasticity of local circuits through different changes (phasic or tonic) in intracellular Ca²⁺.