Hypercalcemia is often associated with a urinary concentration defect. During antidiuresis, organic osmolytes [sorbitol, myo-inositol, taurine, and glycerophosphorylcholine (GPC)] accumulate in the renal inner medulla and are essential for urinary concentration. To clarify the relationship between organic osmolytes and urinary concentration defect in hypercalcemia, examination was made of the effects of hypercalcemia on renal medullary osmolytes content. Rats were put in a state of hypercalcemia by a calcium-rich diet supplemented with CaCO₃ (2.5%/wt) anddaily s.c. injection of I,25(OH)₂ VitD₃ (1.6 μg/kg). They were killed on days 7 and 14. Hypercalcemia induced a urinary concentration defect. Myo-inositol, sorbitol, and GPC contents in the renal medulla were significantly reduced. Aldose reductase activity decreased significantly. Hypercalcemia would thus appear to directly affect renal medullary content of organic osmolytes, thereby modifying renal concentration ability.