We have developed a series of fluorescent Zn2+ sensor molecules with distinct affinities for Zn2+, because biological Zn2+ concentrations vary over a wide range from sub-nanomolar to millimolar. The new sensors have Kd values in the range of 10(-8)-10(-4) M, compared with 2.7 nM for ZnAF-2. They do not fluoresce in the presence of other biologically important metal ions such as calcium or magnesium, and they can detect Zn2+ within 100 ms. In cultured cells, the fluorescence intensity of ZnAF-2 was saturated at low Zn2+ concentration, while that of ZnAF-3 (K-d = 0.79 mu M) was not saturated even at relatively high Zn2+ concentrations. In hippocampal slices, we measured synaptic release of Zn2+ in response to high-potassium-induced depolarization. ZnAF-2 showed similar levels of fluorescence increase in dentate gyrus (DG), CA3 and CA1, which were indistinguishable. However, ZnAF-3 showed a fluorescence increase only in DG. Thus, by using a combination of sensor molecules, it was demonstrated for the first time that a higher Zn2+ concentration is released in DG than in CA3 or CA1 and that we can easily visualize Zn2+ concentration over a wide range. We believe that the use of various combinations of ZnAF family members will offer unprecedented versatility for fluorescence-microscopic imaging of Zn2+ in biological applications.