The use of stable carbon isotopes as a means of studying energy flow is increasing in ecology and paleoecology. However, secondary fractionation and turnover of stable isotopes in animals are poorly understood processes. This study shows that tissues of the gerbil (Meriones unguienlatus) have different δ13C values when equilibrated on corn (C4) or wheat (C3) diets with constant 13C/12C contents. Lipids were depleted 3.0‰ and hair was enriched 1.0‰ relative to the C4 diet. Tissue δ13C values were ranked hair>brain>muscle>liver>fat. After changing the gerbils to a wheat (C3) diet, isotope ratios of the tissues shifted in the direction of the δ13C value of the new diet. The rate at which carbon derived from the corn diet was replaced by carbon derived from the wheat diet was adequately described by a negative exponential decay model for all tissues examined. More metabolically active tissues such as liver and fat had more rapid turnover rates than less metabolically active tissues such as hair. The half-life for carbon ranged from 6.4 days in liver to 47.5 days in hair.The results of this study have important implications for the use of δ13C values as indicators of animal diet. Both fractionation and turnover of stable carbon isotopes in animal tissues may obscure the relative contributions of isotopically distinct dietary components (such as C3 vs. C4, or marine vs. terrestrial) if an animal's diet varies through time. These complications deserve attention in any study using stable isotope ratios of animal tissue as dietary indicators and might be minimized by analysis of several tissues or products covering a range of turnover times.