Explicit internal signal stochastic resonance (EISSR) is investigated numerically when noise is added to the region of periodic-2 oscillatory state in intercellular calcium ion oscillations. It is found that the signal-to-noise ratio (SNR) of strong signal (SS) has a maximum value at an optimal noise intensity and that of weak signal (WS) increases monotonously with the increment of noise intensity. These results indicate that the EISSR has selectivity to two intrinsic signals of system. Moreover, the frequencies of SS and WS hardly change, which implies that system can resist to the effect of noise and sustain intrinsic period oscillatory states. (C) 2004 Elsevier B.V. All rights reserved.