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The Black-tailed tree rat, Thallomys nigricauda, is a southern African xeric-adapted nocturnal rodent which diurally remains in its arboreal nest. Its urine concentrating ability was investigated in a series of dehydration experiments, and kidney morphology was examined. Unlike several other southern African xeric rodents, T. nigricauda did not excrete allantoin precipitate in their urine. However they had a similar efficient urine concentrating ability (UCA) reflected by their urine osmolality and urea concentration when deprived of free water. Renal structure indicated efficient UCA and was also similar to that of other xeric rodents. Because they are unable to maintain body mass on a dry food diet, T. nigricauda require a diet high in preformed water to maintain a constant body mass, despite their efficient UCA. (C) 1998 Academic Press.