The naked mole-rat (Heterocephalus glaber) is a strictly subterranean mammal inhabiting the arid zones of north-east Africa. These animals have no access to free water and water balance thus might be facilitated by regulating renal water loss. The urinary concentrating ability of the naked mole-rat was determined using five dietary manipulations in which both water and salt content were altered. "Control" animals (n = 7) received a high quality protein cereal mixed to a thin paste with water (1 g cereal: 8.5 g water). Water stress was induced by reducing the water content of the diet by either 50% (n = 7) or 65% (n = 7). Salt loading was facilitated by replacing the water with the same volume of either 0.9% salt (n = 7) or 3.0% salt (n=4) solutions. Changes in body mass, food consumption and urine volume were measured daily. The effect of diet on osmolality and electrolyte concentrations of urine and plasma were determined on termination of the diet trials. Although energy intake was not reduced, naked mole-rats lost body weight with both water stress treatments. Urine volume voided per day decreased significantly with both water stress treatments (P<0.05), such that the most extreme water stress led to an 80% reduction in urine volume. Mildly salt-loaded animals gained weight, yet underwent a sodium diuresis, as indicated by a 1.3-fold increase in the daily volume of urine voided (P < 0.05). Maximum urine concentration (1521_+250 mmol.kg -1) was achieved with mild water stress and was 4.6+_0.9 times that of plasma. Neither further water stress nor salt loading further increased urine osmolality (P > 0.05). The naked mole-rat exhibits a moderate kidney concentrating ability and cannot maintain plasma osmolality or body mass with either extreme water stress or salt loading. Although this species successfully inhabits arid zones, survival in these areas is not facilitated by renal water conservation, but rather by their underground existence in a microhabitat where humidities are high and radiant heat loads low. In this