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The Eastern Grey Kangaroo (Macropus giganteus) occurs mostly in the wetter regions of eastern Australia. However, in the past 30-40 years it has moved into more arid regions (rainfall < 250 mm), thus increasing its overlap zone with the xeric adapted Red Kangaroo (Macropus rufus). An increased access to water (supplied for domestic stock) may explain tills range extension, but changes in the availability of preferred feed could also be involved. The water use, drinking patterns and thermoregulatory behaviour of these two species of kangaroo have been examined in a semi-free range study, during Summer at an arid range-land site. Foraging was largely nocturnal in both species arid during the day they behaved to reduce heat loads. This was especially so for M. giganteus, which showed greater shade seeking. However, it Still used more water (72 +/- 2.6 mL kg(-1) day(-1), mean +/- SE) than M. rufus (56 +/- 7.6 mL kg(-1) day(-1)) and drank twice as frequently. Although M. giganteus produced a less concentrated urine (1422 +/- 36 mosmol kg(-1)) than M. rufus, (1843 +/- 28 mosmol kg(-1)), kidney physiology did not explain all of the differences in water metabolism between the species. Water from the feed arid faecal water retention also appear to be involved. Broadly, a better access to reliable water arid the utilisation of mesic microhabitats has enabled M. giganteus to make inroads into the changing rangelands of eastern Australia. However, changes in the vegetation, due to stock grazing, have also favoured M. giganteus, which is a grass eating specialist.