The influence of mineral waters with different calcium content on the urine composition was investigated. Twelve healthy male test persons were housed for 3 x 24 h (once a week) in a climatic chamber under constant resting conditions. The standardized food intake was distributed equally over 24 h. Six 4-h urine samples were collected and chemically analyzed to study the risk of stone formation. The total amount of mineral water (Staatl. Fachingen or Bad Wildunger Helenenquelle) or of tapwater was administered in three portions (700 ml at 9.00, 350 ml at 13.00, 350 ml at 17.00). After mineral water intake, the calcium concentration in the urine was slightly higher. However, the more highly mineralized water caused a significant increase in the urinary magnesium concentration, which persisted into the night. The citrate concentration was increased as a result of alkalization of the urine. After mineral water intake the oxalate excretion was slightly (but not significantly) higher. However, no increased risk of calcium oxalate stone formation was seen when different supersaturation ratios were used as the basis of calculation. In a second study, further 12 volunteers of both sexes were given an oxalate-rich standard meal and 500 ml of the test waters in addition. The urine oxalate concentration was measured hourly for 6 h after the meal, and the total oxalate excretion was calculated. The renal oxalate excretion after mineral water intake was significantly reduced compared with tapwater controls. This effect may be caused by intestinal binding of oxalate by calcium and indicates a further lowering of the risk of calcium oxalate crystallization.