Mammalian skeletal muscles may be classified into two types, fast-contracting (pale muscle) and slow-contracting (red muscles) (Denny-Brown 1929). The motor neurons innervating these two types of muscles can also be distinguished by their electrophysiological properties (Eccles et al. 1958; Kuno 1959; Burke 1967). When the nerves to the two types of skeletal muscles are sectioned and cross-united, the contractile properties of the muscles are partly transformed following reinnervation (Buller et al. 1960; Buller and Lewis 1965). Therefore there seems little doubt that the different properties of the two types of muscles are induced, or at least maintained, by an influence of the innervating motor neurons.