The renal corpuscles of desert deermice and swamp deermice were investigated for quantitative differences in structure by statistico-geometrical (stereological) methods. There was found to be essentially no difference between the two groups in mean glomerular size. Blood channel length per unit glomerular volume in swamp mice was found to be 1.09 times that of desert mice. Filtration surface area per unit glomerular volume in swamp mice was found to be 1.25 times that of the desert mice. However, these relatively small differences were greatly magnified by a 1.6-fold difference in the number of glomeruli per kidney in swamp mice (13,700) over desert mice (8,600). As a result, the blood channel length per kidney in swamp mice (23 m) was 1.77 times that in desert mice (13 m), and filtration surface area per kidney in swamp mice (430 mm<sup>2</sup>) was found to be 2.05 times that of desert mice (210 mm<sup>2</sup>).