A variety of cell types respond to extracellular hormonal signals by repetitive intracellular calcium spikes. Here, by constructing a mesoscopic stochastic model for intracellular calcium oscillations in hepatocytes, we have investigated the influence of internal noise on the detection of sub-threshold hormonal signals using chemical Langevin equation (CLE) and Poisson approximation. It is found that stochastic calcium spikes appear when the internal noise is considered, and the regularity of the spike train undergoes a maximum with the variation of the internal noise level, indicating the occurrence of internal noisestochastie resonance (INSR). Since the magnitude of the internal noise is changed via the variation of the system size, the INSR also represents itself a kind of system sizc resonance. (C) 2004 Published by Elsevier B.V.