The development of molecular biology techniques (which enable the modification and expression of exogenous cDNA in heterologous cell types), has been responsible in recent years for the widespread use of protein probes by cell biologists, for the measurement of biological parameters. To this end, two main types of proteins are used, isolated from a wide variety of luminescent organisms. The first group consists of chemiluminescent proteins. These are proteins that emit light, often associated to physiological parameters of interest, such as changes in ATP or Ca2+ concentration. Since mammalian cells do not possess endogenous lumiunescent molecules, the use of these proteins is normally associated to an excellent signal-to-noise ratio, as exemplified by aequorin, a photoprotein isolated from Aequorea victoria. The second group consists of fluorescent proteins. Among these, Aequorea victoria's "Green Fluorescent Protein" (GFP) has attracted much attention, since its expression (or that of fusion products with proteins of interest) generates an intense signal, which enables complex phenomena (such as organelle structure and their dynamic changes, protein targeting, etc) to be followed in real time in living cells. This review presents some of the results obtained using aequorin for studying intracellular Ca2+ homeostasis.