Mammalian species of large stature have larger kidneys with larger nephrons than smaller animals, although this relationship is not linear. In order to investigate whether in animals of different sizes within the same species similar differences exist, a study in mongrel dogs was undertaken. In animals weighing between 9 and 42 kg with kidney weights from 25 to 79 g the area and perimeter of the glomeruli and the cortical width were measured. The number of glomeruli per unit area was counted and corrected for the size of the glomeruli. The results show that larger dogs have larger kidneys and that this enlargement is due to the increase in size of the nephrons and not in their number.