The invention relates to a method for optical detection of the dynamics of Ca.sup.2+ in a biological system, said method comprising monitoring the photons emitted by a recombinant Ca.sup.2+-sensitive polypeptide, which comprises or consists of a chemiluminescent protein linked to a fluorescent protein, present in said biological system. In a particular embodiment, said recombinant polypeptide comprises or consists of the Aequorin and GFP linked by a linker allowing chemiluminescence resonance energy transfer (CRET), and optionally comprises a peptidic fragment capable of targeting said recombinant polypeptide into a specific cellular domain or compartment. The present invention also concerns a transgenic non-human animal expressing said recombinant polypeptide sensitive to calcium concentration, in conditions enabling the in vivo monitoring of Ca.sup.2+ dynamics. In a particular embodiment, the expression and/or localization of said recombinant polypeptide is restricted to a specific tissue, a single-cell type and/or in a particular cellular compartment or domain.