Little is known about regulation of ion channels during early development. Ascidian embryos (Halocynthia roretzi) serve as a model system to address this issue because of its simple embryogenesis. Here we studied development of calcium channels in cleavage-arrested neuronal and muscular blastomeres with the modified two-electrode voltage clamp method in which CsCl/EGTA was used in the low-resistance current-injection electrode. Activation kinetics and I-V relation of Ba 2 + currents were similar between the two types of cell. In both cells, low level of Ba 2 + current was first detected at the stage prior to the emergence of the delayed-rectifier K + channel current. Expression of TuCa I, an ascidian homologue of L-type Ca 2 + channel alpha1-subunit, was detected in neurons and muscle cells by the RT-PCR and the whole-mount in situ hybridization. These results indicate that the same Ca 2 + channel alpha1-subunit gene is commonly regulated in ascidian neurons and muscle cells.