1. We studied urine concentrating ability in ten magnesium-depleted puppies and compared the results with those of match-fed, littermate controls.

2. The experimental puppies became hypomagnesaemic and hypocalcaemic without evidence of potassium depletion. After 24 h of food and water deprivation, urine osmolality increased to 1350 ± 340 mosm/kg of water. This value did not differ from that of control animals.

3. We also studied urine concentrating ability in a child with primary hypomagnesaemia and secondary hypocalcaemia. He could achieve a urine concentration of 1080 mosm/kg of water after 20 h of water deprivation when he was hypomagnesaemic and hypocalcaemic.

4. We conclude that urine concentrating ability is normal in magnesium depletion uncomplicated by hypercalcaemia or severe potassium depletion.

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