In this study, an efficient microtechnique (gel crystallization method) was used to investigate the in-vivo effect of sodium-potassium citrate on the crystal growth rate of calcium oxalate (Vcr) in human urine samples of 6 healthy volunteers. With a daily dose of 3 X 11 mmol of alkali citrate, Vcr decreased by 70%. This could have been due to the decrease of calcium excretion, which caused 50-60% of the total change, and to the increase of citrate and pH, each contributing about 20-25% to the decline of Vcr. The findings explain the clinical advantages of alkali citrates in the prevention of recurrent calcium oxalate stone formation.