Urinary concentrating ability was tested under protein and salt load in the four chromosomal species of subterranean mole-rats (Spalax ehrenbergi superspecies) found in Israel. Protein stress induced by a diet of soyabeans supplemented by agar gel, demonstrated a significant increase in urinary osmolarity (UO). In the species living in the driest and warmest region (2n = 60). UO (1423·101 mOsmol/kg) was significantly (P<0·05) higher than in the other three species (2n = 52. 1172·31 mOsmol/kg; 2n = 54, 1160·116 mOsmol kg; and 2n = 58, 1216·145 mOsmol/kg). Upon salt loading this diet with 0·3 mol NaCl, UO increased significantly. However, when the salt load was increased to 0·45 mol NaCl, UO decreased significantly in all but one species (2n = 60) which maintained UO at 1522·65 mOsmol/kg. A decline in UO was attributed to diuresis resulting from a significant increase in urine and sodium excretion. The kidney, of only the xeric ranging species (2n = 60), demonstrated the ability to produce a hyperosmotic urine, in spite of the high salt load. These results might explain the restricted distribution of S. ehrenbergi: the only species (2n = 60) found in an environment rich in succulents and halophyte plants (steppe). This species appears to push speciation and adaptive radiation to the southern limit of its superspecies.