1. In controlled laboratory experiments, rectal temperature, cutaneous moisture evaporation, and respiratory rate were studied in the dik-dik. The water balance of the dik-dik was investigated when the antelope were either fully hydrated or dehydrated at the environmental temperature of 22 degrees C or at temperatures alternating between 22 degrees C and 40 degrees C at 12 h intervals. 2. Faecal, urinary and evaporative water losses were all reduced by varying degrees during dehydration at 22 degrees C and 22 to 40 degrees C. The highest urine osmolality recorded was 4318 ± 105 mosmol/kg H2O which occurred when the antelopes were severely dehydrated. 3. At the high air temperatures (40 to 45 degrees C) cutaneous evaporation measured with a non-ventilated sweat capsule was 19 g H2O m-2 h-1 and the respiratory rate over 360/min. Thus the respiratory tract seems to be the major avenue for dissipating excess heat in dik-dik exposed to thermal stress. Injection of adrenaline, intravenously, stimulated sweat discharges similar to those observed in the small gazelles, and sheep and goats. 4. The low-water exchange and an efficient kidney helps explain the ability of this small antelope to inhabit hot arid areas.