Two Damara and two South African Mutton Merino wethers of the same age were deprived of water for seven days under high environmental temperatures. During this time the body mass of both breeds decreased by about 29% and daily feed consumption and total faecal water declined between 97 and 99%. Urine volume decreased similarly in both breeds but the osmolarity of the Damara urine reached a mean peak of 2836 mOsm/1 while the Mutton Merinos only reached 1719 mOsm/1. This difference was reflected in plasma osmolarity with the mean for the Damaras reaching 312 mOsm/1 and the Mutton Merinos 334 mOsm/1. Furthermore the packed cell volume (PCV) in the Damaras increased only 17 % while in the Mutton Merinos it increased 28%. These differences suggest that the Damara, due mainly to its superior renal concentrating ability, is better adapted than the Mutton Merino to dehydrational stress in a hot environment.