Alkalizing beverages are highly effective in preventing the recurrence of calcium oxalate (Ox), uric acid and cystine lithiasis. The aim of the present study was to evaluate the influence of grapefruit-juice and apple-juice consumption on the excretion of urinary variables and the risk of crystallization in comparison with orange juice. All investigations were carried out on nine healthy female subjects without any history of stone formation and aged 26-35 years. Each juice was tested in a 5 d study. During the study, the subjects received a standardized diet. Fluid intake of 2.75 litres was composed of 2.25 litres neutral mineral water, 0.4 litre coffee and 0.1 litre milk. On the fourth and fifth day 0.5 litre mineral water was partly substituted by 0.5 or 1.0 litre juice under investigation respectively. The influence on urinary variables was evaluated in 24 h urine samples. In addition, the BONN risk index of CaOx, relative supersaturation (RS)CaOx crystallization was determined. Due to an increased pH value and an increased citric acid excretion after consumption of each juice, the RSCaOx decreased statistically significantly (P<0.05) for grapefruit juice, but not significantly for orange and apple juice. The BONN risk index yielded a distinct decrease in the crystallization risk. We showed that both grapefruit juice and apple juice reduce the risk of CaOx stone formation at a magnitude comparable with the effects obtained from orange juice.