Variations in calcium ion (Ca2+) concentrations play a crucial role in cell-cell communication; periodic changes act as cellular signals. In this study, we developed a solid-state calcium ion-selective electrode (Ca2+ ISE) integrated in a microchannel to measure extracellular Ca2+ variations; an ISE has a high detection sensitivity (as low as 10(-9) M) and selectivity against potassium ions (K+). The sensor was used to monitor the cellular activities of HepG2/C3As cultured in a microchannel. We measured a periodically oscillating extracellular Ca2+ concentration that was superimposed on a gradual increase in the Ca2+ concentration after potassium chloride (KCl) stimulation. The oscillation period was in the range of 30-50 s, which was in good agreement with the literature.