We tested the hypotheses that relative medullary thickness (RMT) and kidney mass are positively related to habitat aridity in rodents, after controlling for correlations with body mass. Body mass, mass‐corrected kidney mass, mass‐corrected RMT, mass‐corrected maximum urine concentration, and habitat (scored on a semiquantitative scale of 1–4 to indicate increasing aridity) all showed statistically significant phylogenetic signal. Body mass varied significantly among habitats, with the main difference being that aquatic species are larger than those from other habitats. Mass‐corrected RMT and urine concentration showed a significant positive correlation (N = 38; conventional r = 0.649, phylogenetically independent contrasts [IC] r = 0.685), thus validating RMT as a comparative index of urine concentrating ability. RMT scaled with body mass to an exponent significantly less than 0 (N = 141 species; conventional allometric slope = −0.145 [95% confidence interval (CI) = −0.172, −0.117], IC allometric slope = −0.132 [95% CI = −0.180, −0.083]). Kidney mass scaled to an exponent significantly less than unity (N = 104 species; conventional slope = 0.809 [95% CI = 0.751, 0.868], IC slope = 0.773 [95% CI = 0.676, 0.871]). Both conventional and phylogenetic analysis indicated that RMT varied among habitats, with rodents from arid areas having the largest values of RMT. A phylogenetic analysis indicated that mass‐corrected kidney mass was positively related to habitat aridity.