The calcium ion, a second messenger in the brain, plays key roles in neuronal signaling pathways. Ca(2+) signals in neurons are often highly localized and difficult to measure accurately. The quantification of calcium concentration is thus critical for understanding neuronal signaling. In this study, a yellow cameleon (YC3.60) excited using a 458 nm laser was used to monitor the calcium signals in neurons, and the dynamic range (R(max)/R(min)) of YC3.60 was found to reach 250%. The spatial distribution of calcium and the physiological changes in hippocampal neurons and even in spines were determined by the fluorescence resonance energy transfer (FRET) method. It was proved that cameleon could be used for the quantitative measurement of calcium concentration in neurons. Fluorescence readout of the calcium concentration in neurons by FRET is nondestructive, quantifiable with high spatiotemporal resolution, and even applicable to dendritic spines.