The effect of prolonged bedrest immobilization on urinary risk factors for stone formation and on the propensity for the crystallization of calcium salts was examined in eight normal subjects. During 5 weeks of bedrest, the mean urinary calcium excretion rose during the first week and remained elevated (from 5.68 to approximately 7.50 mmol/day). Mean urinary phosphorus excretion increased by the second week of bedrest and remained elevated (from 2.70 to approximately 30.6 mmol/day). Urinary sodium and uric acid excretion rose slightly, as did urinary magnesium. Urinary pH, oxalate, and citrate changed slightly or not at all. Owing to these biochemical alterations, urinary saturation of calcium phosphate, calcium oxalate, and monosodium urate increased significantly during bedrest, but that of uric acid did not change. The inhibitor activity against the spontaneous nucleation of brushite (CaHPO4.2H2O) and calcium oxalate was not altered significantly by bedrest. Thus, the propensity for the crystallization of stone-forming calcium salts was enhanced by bedrest, suggesting that immobilization may confer increased risk for the formation of calcium-containing renal stones.