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The aim of this paper was to analyze the renal performance of the insectivorous desert mouse opossum, Thylamys pusilla, a marsupial that inhabits temperate drylands of the Neotropics. We measured the urine concentrating ability of this marsupial in the field and under three different experimental conditions in the laboratory. (1) Ad libitum treatment: animals were fed ad lib. on apples, sausages and cat food; (2) restriction treatment: foods high in nitrogen were supplied ad lib., while only apple, a food with high water to nitrogen load, was restricted, and (3) fasting: 2 days of food deprivation. Osmolarity values recorded in the field were similar to those obtained under the restriction treatment. The maximum urinary concentration found (9015 mOsm/l) is the highest reported for a marsupial and more in keeping with similarly sized desert rodents. Similar urine concentrations in T. pusilla and desert rodents suggest that xeric environments, rather than phylogeny, play a major role in water conservation. The kidney of T. pusilla possesses a long papilla and shows high renal indices (M/C: 9.37; RMT: 11.76). Our results question the idea that urine concentration is not high in desert-dwelling insectivores. Although their dietary water intake is potentially high, this may not always be sufficient to eliminate the nitrogenous waste resulting from insectivory. Thus, T. pusilla uses its ability to conserve water to overcome periods of food shortage and high nitrogen load. (C) 2001 Elsevier Science Inc. All rights reserved.