Whereas Kolwe described and measured pronation and supination in the cat as early as 1920, the present study deals with rotation of the lower limb of the cat and dog from both comparative and functional aspects. Active supination predominates in cats during climbing and the capture of prey; in the canine passive supination and rotation of the lower limb are used in conjunction with directional changes involved in the chase. The difference in the extent of inward rotation between long- and short-legged breeds of dogs underscores this hypothesis.