A confocal spot detection optical setup was used to record fluorescence signals in response to calcium pulses, elicited by flash photolysis of DM-nitrophen, with the calcium indicators CaOrange-5N and Fluo-3. Our results yield the following conclusions: [Ca2+] changes are almost perfect spikes at pCa 9 and broader transients followed by a step at pCa 7. The [Ca2+] spikes were used to measure the dissociation rate constant of the Ca2+ dyes. Experiments at pCa 7 were used to verify the kinetic rate constants of the dyes and to obtain those of DM-nitrophen. The association rate constant of this compound was found to be more than one order of magnitude faster than that suggested previously. CaOrange-5N was able to track changes in [Ca2+] more accurately than Fluo-3. This latter dye introduced severe distortions which preclude a quantitative deconvolution of the fluorescence transients into changes in the free [Ca2+].