To understand the genesis of urinary stones different crystallization models have been developed to simulate the crystallization processes that occur at the beginning of the stone formation. In this study a laser probe working on the principle of the measurement of back-scattered light was mounted on a batch crystallization model, in which calcium oxalate nucleation was induced by titration of an artificial urine with ammonium oxalate. With the laser probe the particle numbers and particle size distributions in the range from 1-250 microm can be measured in a single experiment. Measurement is performed directly at the place of crystal development and, therefore, falsification of the results (by dilution, mechanical transport or isolation, temperature gap, multiple measurements, mathematical calculations after indirect measurements, etc.) is prevented. In a basic urine, containing 7.5 mmol/l calcium (4.13 mmol/l free calcium), nucleation of calcium oxalate appears at a concentration of 0.58 mmol/l oxalate. At the start of nucleation the particle size rises to 7 microm. After addition of citrate and magnesium in concentrations up to 12 mmol/l the metastable limit is clearly shifted up to 1.50 mmol/l oxalate. Both findings are in good agreement with the literature. The laser crystallization model has proved to be useful for the testing of inhibitors and promoters of urinary stone formation.