Lindeborg (1952) was the first to measure water consumption of both _P. m. rufinus_ and _P. t. truei_, the species and subspecies with which my experiments are concerned. Lindeborg also tested the ability of five races of _Peromyscus_ to survive reduced water rations. Unfortunately, the subspecies chosen for these experiments did not include _P. t. truei_ or _P. m. rufinus_. Lindeborg (1952:25) found that the "amounts of water consumed by various species of _Peromyscus_ from different habitats within the same climatic region were not conclusively different." However, he did find significant differences between some subspecies from different geographical areas. For example, he found no significant difference in water consumption between _P. m. bairdii_ from Michigan and either _P. m. blandus_ or _P. m. rufinus_ from New Mexico, but he found a highly significant difference between _P. l. noveboracensis_ from Michigan and _P. l. tornillo_ from New Mexico. Lindeborg also found that the subspecies of _Peromyscus_ that consumed the least water, and that were best able to survive a reduced water ration, were those from the more xeric climatic areas. Some mammals may be able to change their diets in times of water stress, and thereby compensate for a shortage of water. At such times, _Dipodomys_ selects foods with high percentages of carbohydrates and conserves water by reducing the amounts of nitrogenous wastes to be excreted (Schmidt-Nielsen _et al._, 1948). Williams (1959b) found that _P. m. osgoodi_ from Colorado drank more water on a diet rich in protein than on one rich in carbohydrates. But, her mice on a high carbohydrate diet used less than a normal amount of water for a period of only five weeks; at the end of the five weeks they were drinking about as much as they had been when on the control diet of laboratory chow. Likewise, mice adjusted to the high protein diet by consuming more water; but by the end of the fifth week their daily water consumption approximated the amount drunk when fed on laboratory chow. Because of these results, Williams questioned the validity of the assumption that _P. maniculatus_ is able to inhabit a diversity of habitats because of its adaptability with respect to food and water requirements. I conducted a series of experiments on water and food consumption by individuals of _P. truei_ and _P. maniculatus_. It was thought that if there were differences in water or food consumption, or both, knowledge of them might help to explain the obvious differences in habitat preferences of these two species in Mesa Verde National Park. In August of 1965, 30 individuals of _P. truei_ and _P. maniculatus_ were trapped in Mesa Verde National Park at elevations of 7000-8400 feet, and transported to Lawrence, Kansas, where the experiments were carried out.