Calcium specific fluorescence probes such as Quin-2 and Fura-2 have been proved to have a few defects in determining cytosolic free $Ca^{2+}$ in cells (Hallett et al., 1990; Callaham et al., 1991; Steinberg et al., 1987; Kao et al., 1989; Minta et al., 1989; Zee et al., 1989). In this report, a new probe Fluo-3/AM was employed to determine cytosolic free $Ca^{2+}$ concentration in oat cells and it was identified not to be penetrated into the intracellular calcium store (vacuole). Phytochrome action in changing the cytosolic free$Ca^{2+}$ concentration in oat cell was investigated by irradiating red and far-red lights to isolated oat protoplasts. Red light irradiation (fluence rate : $320{mu}mol{cdot}m^{-2}{cdot}s^{-1}$) for 30 sec increased the cytosolic free $Ca^{2+}$ from 101 nM to 115 nM, but far-red light (fluence rate : $427{mu}mol{cdot}m^{-2}{cdot}s^{-1}$) did not. Photoreversibility of phytochrome in changing cytosolic $Ca^{2+}$ concentration was clearly observed. Mobilization of $Ca^{2+}$ from the intracellular calcium store to the cytoplasm by 5 min irradiation of red light was observed to be 6 nM under conditions that external $Ca^{2+}$ were removed by excess EGTA, and that calcium channels of the plasma membrane were blocked by their blockers (verapamil and lanthanum ion). These results imply that Pfr form of phytochrome functions in the process of influx of $Ca^{2+}$, but also in the mobilization process of $Ca^{2+}$ from its pool in the cell.