Dusky-footed woodrats (Neotoma fuscipes) and desert woodrats (N. lepida) experience pronounced seasonal variations in the aridity of their habitats. The effects of seasonal aridity upon the water conserving abilities of these species were assessed through measurements of water conserving abilities and kidney structure of animals captured in summer and winter, and through measurements of animals' abilities to acclimate to differing water availabilities in the laboratory.Urine concentrating ability was the water conserving mechanism most responsive to changes in the availability of water. Summer and summeracclimated N. fuscipes (431.7 and 459.4 mEqCl/1) demonstrated urine Cl concentrating abilities substantially greater than those of winter and winter-acclimated N. fuscipes (245.7 and 337.4 mEqCl/1). Summer, winter-acclimated, and winter N. lepida exhibited urine Cl concentrations equivalent to those of winter N. fuscipes; summer-acclimated N. lepida exhibited markedly greater values (466.7 mEqCl/l) equivalent to those of summer and summer-acclimated N. fuscipes.Measurements of relative thicknesses of renal cortex and medulla yielded no significant differences among the experimental groups of N. fuscipes and N. lepida, thus suggesting that both species possess equal abilities to concentrate urine. These data are confirmed by urine concentrations of summer-acclimated animals of both species.Water conserving abilities of both species correlate well with climatic and dietary plant water content data. Thus, during the dry, warm summer months (when plant moisture is reduced) N. fuscipes conserves water mainly through increased urine concentration. The laboratory acclimation data and differences between summer and winter animals strongly suggest that N. fuscipes undergoes an acclimatization to the seasonal aridity which increases gradually during spring and peaks in late summer, thus enabling this water-dependent species to exist on reduced water requirements. The uniformly low water conserving abilities of winter-acclimated, winter, and summer N. lepida physiologically verify the previous reports that this species satisfies its water requirements through utilization of succulent cactus, thereby avoiding the stress of summer aridity in its habitat.The fact that both species exhibit equal capacities to conserve water indicates that the much greater geographic distribution of N. lepida is not the result of differences in physiological water conserving abilities, but instead may be the result of specific physiological adaptation by N. lepida to utilization of cactus and other plants containing noxious or toxic compounds.