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Carbon and oxygen isotope ratios of bone apatite were measured in 14 endothermic and ectothermic vertebrates native to the Chihuahuan Desert and collected in June and July of 1999 and 2000. The delta(18)O values of most reptiles were very high, up to 44parts per thousand (standard mean ocean water (SMOW), some of the highest delta(18)O values ever measured for an animal. The delta(18)O values of rodents and birds were lower (similar to 32 +/- 5parts per thousand vSMOW), and the earless lizard Holbrookia maculata were the lowest of all species analysed (similar to25parts per thousand). Omnivorous grasshopper mice (Onychomys torridus) had lower delta(18)O values than granivorous rodents. Results from oxygen analysis likely reflect variation in diet and body water flux differences between endotherms and ectotherms. Carbon isotope analysis revealed a dramatic shift in diet from C-3 plants in 1999 to C-4 plants in 2000 in most rodents and birds. Kangaroo rats and reptiles did not change, having a constant delta(13)C value indicative of a C-3-based diet in both years. This suggests reliance on winter annual plant seed caches for kangaroo rats, but not other rodents. The carbon isotope data can be explained in terms of seasonal differences within and between years in the timing and intensity of the seasonal rainfall events, and the productivity of summer and winter annual plants. This study illustrates that stable isotope analysis is a powerful method for tracking dietary change and feeding behavior in desert vertebrates. (C) 2002 Elsevier Science Ltd..