Sonoran Mud Turtles (Kinosternon sonoriense) are widely believed to be restricted to permanent bodies of water, yet some populations apparently thrive in canyons that dry completely. We investigated the possibility that these turtles could estivate by subjecting individuals collected in the Peloncillo Mountains of New Mexico to 11 weeks without food or water. Turtles apparently used water stored in the urinary bladder to osmoregulate early in the period of deprivation. However, by the end, several blood chemistry parameters (including plasma osmolality, sodium and potassium concentrations, and blood urea nitrogen) had increased anhomeostatically to very high levels; most variables returned to original levels on rehydration. Hematocrit and plasma protein levels were unaffected, suggesting preferential defense of plasma volume by dehydrating turtles. Individuals varied in their burial behavior, and individuals that were surface active became more dehydrated and accumulated more urea than those that remained buried. We were unable to demonstrate metabolic depression or a decrease in evaporative water loss (EWL) during the trial, but rates of resting metabolism and EWL in K. sonoriense were similar to those of Oklahoma-collected K. flavescens (a species well known to estivate) treated identically. Kinosternon flavescens produced less nitrogenous waste than did K. sonoriense and may have been more uricotelic but also incurred more severe hypernatremia. All but one K. sonoriense recovered upon return to water. We conclude that Sonoran mud turtles from this population are physiologically capable of surviving many weeks without food and water. The concept of estivation as a special physiological state (as opposed to a behavior) is critically discussed.