In hepatocytes, proliferation of tetraploid and binuclear cells and an increase in cytosolic and mitochondrial protein synthesis are caused by He-Ne laser irradiation. To gain some insight into the mechanism of photomodulation of cellular and subcellular activities in isolated hepatocytes, intracellular mediators of cell photostimulation were investigated in intact cells and isolated mitochondria. Irradiation of isolated hepatocytes and isolated rat liver mitochondria was carried out with He-Ne laser (wavelength: 632.8 nm; fluence: 0.24 J cm-2; fluence rate: 12 mW cm-2). Changes in cytosolic [(Ca2+)c] and mitochondrial [(Ca2+)m] calcium concentration, and in mitochondrial (Δψm) and plasma (Δψc) membrane potential were, monitored using either colorimetric or fluorescent probes. C-fos expression was studied by Northern and immunoblotting analysis. As a result of irradiation, an increase in (Ca2+)c and a calcium-dependent increase in Δψc were found. The increase in (Ca2+)c, in turn, caused an increase in c-fos expression. Finally, an increase in (Ca2+)m, probably owing to the increase in Δψm, was found. Increase in (Ca2+)c, leading to activation of gene expression, and a general activation of mitochondrial metabolism, could play a major role in the mechanism of photostimulation of cellular activities by He-Ne laser.