The development of fluorescent calcium (Ca2+ ) indicators has been a very powerful tool for looking at the role of this second messenger in signal transduction. The ability to load these indicators into cells in a nondisruptive manner allow for the visualization of intracellular-free Ca2+ ion ([Ca2+ ]i ) in a living developing embryo (reviewed in refs. 1 and 2 ). The vertebrate zebrafish (Danio rerio ) is ideally suited for fluorescent studies by virtue of its transparent embryos. The zebrafish has two phases of [Ca2+ ]i release in the first few hours of development. The first is a phase of dramatic long-lived [Ca2+ ]i elevations associated with the forming cleavage furrows during the first few cell divisions (3 ). After the 16–32-cell stage, a subset of cells displays rapid aperiodic [Ca2+ ]i increases, localized to the enveloping region (4 ), and persist until the 1000–2000-cell stage (4 ,5 ) (see Fig. 1 ).