Eleven hyponatremic dogs were unable to concentrate their urine during periods of severe dehydration and azotemia. When normonatremia was reestablished in eight of the dogs, their renal concentrating ability returned. Six dogs, including the 3 dogs in which normonatremia was not reestablished, died or were euthanatized; renal lesions were not found during postmortem examination. Two dogs had hypoadrenocorticism, which has been documented as a cause of hyponatremia and impaired renal concentrating ability. Two dogs had gastrointestinal disease, which has been documented as a cause of hyponatremia, but not of impairment of renal concentrating ability. All dogs without hypoadrenocorticism had clinical and clinicopathologic indications of blood loss, which has not been documented as a cause of hyponatremia or impairment of renal concentrating ability. Hyponatremia (less than 120 mEq/L) was induced by chronic blood removal in a dog maintained on a low-sodium diet. During the period of hyponatremia, the dog became azotemic, hypotensive, and severely dehydrated; renal concentrating ability was impaired. We conclude that hyponatremia may be caused by hemorrhage, but irrespective of the cause, hyponatremia impairs renal concentrating ability.