Monitoring of crystallization of calcium salts with ion-selective electrodes has turned out to be a very sensitive method. The difficulties of handling these electrodes in native whole urine and other biological fluids have been eliminated by new calcium analyzers, which clean and calibrate the electrodes after each measurement. To study crystallization kinetics, repeated calcium ion measurements have to be performed at regular intervals. For this purpose we have developed a special sampler and software. The sampler brings a thermostat-controlled crystallization chamber to the analyzer at preselected intervals. The computer directs and coordinates the sampler and the analyzer, stores the received results and prints out growth curves. Furthermore it calculates the half-time (h) and the maximum decrease of ionic calcium at infinite incubation time (delta Ca2+ infinity). Both values are shown to characterize the growth of calcium oxalate monohydrate in urine. Results are obtained within 40 min.