The mechanisms for the formation of calcium oxalate kidney stones are not well understood. In our urological basic research study, ammonium oxalate was added to sample solutions of CaCl2 and to artificial urines to induce the formation of calcium oxalate crystals, the most common crystals in the urinary tract. Using an in-line laser-probe we took real-time readings (4 min intervals) of the PSD evolution of particles ranging in size between 0.5 and 250 mum over a period of 1 h. Most particles were between 0.5 and 14 mum in size. The particle-size mode is approximately 2 mum. The time-dependent PSD development depends on the composition of the solution. The in-line laser-probe permits individual analysis of minute particles which fall below the lower coulter counter detection limit. As these particles form the basis of all further processes which alter the particle size, they are of paramount interest in urinary stone research.