Calcium signals play an important role in a variety of processes necessary for neuronal development. Whilst the characteristics and function of calcium signals have been comprehensively examined in vitro, the significance of these signals during development in an intact embryo remains unclear. In this study, we have examined the spatial and temporal patterns of intracellular calcium signals in precursor cells (cells without processes) within the spinal cord of the intact zebrafish embryo aged between 17 and 27 h. In total, approximately one-third of cells displayed spontaneous intracellular calcium transients. The calcium transients had an average peak amplitude of 33.3 (+/-2.8%) above baseline, a duration of 52.2 (+/-6.3 s) and occurred with an average frequency of 4.6 (+/-0.4 per hour). Calcium transients were observed in precursor cells located throughout the spinal cord, with the highest percentage of active cells (35.1+/-8%) occurring at a developmental time of 21-22 h. Furthermore these intracellular calcium signals were observed in the presence of tricaine, indicating that they are not generated via sodium-dependent action potentials. In precursor cells loaded with the calcium buffer BAPTA both the frequency and the amplitude of the calcium transients was significantly reduced. The intracellular calcium transients may represent a common activity-independent calcium-mediated mechanism that contributes to the regulation of neuronal development in the spinal cord of the zebrafish embryo during the segmentation and early pharyngula period.