To visualize Ca²⁺-dependent protein–protein interactions in living cells by fluorescence readouts, we used a circularly permuted green fluorescent protein (cpGFP), in which the amino and carboxyl portions had been interchanged and reconnected by a short spacer between the original termini. The cpGFP was fused to calmodulin and its target peptide, M13. The chimeric protein, which we have named “pericam,” was fluorescent and its spectral properties changed reversibly with the amount of Ca²⁺, probably because of the interaction between calmodulin and M13 leading to an alteration of the environment surrounding the chromophore. Three types of pericam were obtained by mutating several amino acids adjacent to the chromophore. Of these, “flash-pericam” became brighter with Ca²⁺, whereas “inverse-pericam” dimmed. On the other hand, “ratiometric-pericam” had an excitation wavelength changing in a Ca²⁺-dependent manner. All of the pericams expressed in HeLa cells were able to monitor free Ca²⁺ dynamics, such as Ca²⁺ oscillations in the cytosol and the nucleus. Ca²⁺ imaging using high-speed confocal line-scanning microscopy and a flash-pericam allowed to detect the free propagation of Ca²⁺ ions across the nuclear envelope. Then, free Ca²⁺ concentrations in the nucleus and mitochondria were simultaneously measured by using ratiometric-pericams having appropriate localization signals, revealing that extra-mitochondrial Ca²⁺ transients caused rapid changes in the concentration of mitochondrial Ca²⁺. Finally, a “split-pericam” was made by deleting the linker in the flash-pericam. The Ca²⁺-dependent interaction between calmodulin and M13 in HeLa cells was monitored by the association of the two halves of GFP, neither of which was fluorescent by itself.