_P. truei_ may be restricted to the pinyon-juniper woodland because of its need for more mesic conditions. Still, Mesa Verde is semi-arid and there are few permanent sources of water available for animals. The primary source of moisture for rodents must be their food. Analysis of the percentages of moisture contained in the three most common plants in the trapping grid showed that _P. truei_ could obtain the required moisture by eating about ten grams of these plants daily; individuals of _P. maniculatus_ would need to eat less in order to satisfy their water needs. Individuals of _P. truei_ died more frequently in warm live-traps than did individuals of _P. maniculatus_. This indicates that _P. truei_ can tolerate less desiccation, or a narrower range of temperatures, than can _P. maniculatus_. Both species of mice eat some of the same plants, but these plants occur widely. _P. truei_ seems to rely more upon the nuts of pinyons and the seeds of junipers than does _P. maniculatus_. Mounds of discarded juniper seeds were associated with all nesting sites of _P. truei_. Bailey (1931:153) also noticed the fondness of this species for pine nuts and juniper seeds. Apparently, the availability of these foods is one of the major factors affecting the distribution of _P. truei_. However, this is not the only factor, as is shown by the presence of _P. maniculatus_ but lack of _P. truei_ in a juniper-pinyon association with an understory of bitterbrush. This habitat was seemingly too arid for _P. truei_. Factors Affecting Population Densities The production of young, and success in rearing them, is essential to continuity of any population. _P. maniculatus_ is favored in this respect, because the females produce more young and wean them sooner than do females of _P. truei_. In addition, lactating females of _P. maniculatus_ require significantly less water than do females of _P. truei_. Since young mice of both species require no more water per gram of body weight than do adults, the young can disperse into any area that is habitable by their species. _P. maniculatus_ probably is affected less by prolonged drought than is _P. truei_. Since lactating females require the most water of any animal in the population, they are the weakest link in the system. Females of _Peromyscus_ are known to reabsorb embryos when conditions are unfavorable for continued pregnancy. If prolonged drought occurred in the reproductive season, and desiccated the vegetation upon which the mice depend for moisture, the populations should diminish the following year. Lactating females of _P. truei_ would be affected more seriously by a shortage of water than would lactating females of _P. maniculatus_.