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Temporal stable isotope records derived from animal tissues are increasingly studied to determine dietary and climatic histories. Despite this, the turnover times governing rates of isotope equilibration in specific tissues following a dietary isotope change are poorly known. The dietary isotope changes recorded in the hair and blood bicarbonate of two adult horses in this study are found to be successfully described by a model having three exponential isotope pools. For horse tail hair, the carbon isotope response observed following a dietary change from a C₃ to a C₄ grass was consistent with a pool having a very fast turnover rate (t _1/2~0.5 days) that made up ~41% of the isotope signal, a pool with an intermediate turnover rate (t _1/2 ~4 days) that comprised ~15% of the isotope signal, and a pool with very slow turnover rate (t _1/2 ~140 days) that made up ~44% of the …