Abstract: The synthesis and compound of a new caged calcium which is an ortho-nitrophenyl derivative of EGTA and various intermediates. It is synthesized in ten steps and 24% overall yield. The photosensitive chelator, nitrophenyl-EGTA, has a K.sub.d for Ca.sup.2+ of 80 nM and for Mg.sup.2+ of 8.8 mM. Upon exposure to ultra-violet light (in the region of 350 nm), the chelator is cleaved, yielding iminodiacetic acid photoproducts with known low Ca-affinity (K.sub.d =1 mM). The quantum yield of photolysis of nitrophenyl-EGTA in the presence of Ca.sup.a+ is 0.23 and in the absence of Ca.sup.2+ is 0.20. In experiments with chemically skinned skeletal muscle fibers, a fully relaxed fiber equilibrated with nitrophenyl-EGTA:Ca.sup.2+ complex, in the presence of physiological [Mg.sup.2+ ] (i.e. 1.0 mM), produced maximal contraction after a single flash from a frequency doubled ruby laser (347 nm). Half-maximal tension was achieved in 18 ms at 15.degree. C.