Observations in cats of flexion and extension movements of the 4 limbs have led to the conclusion that the different forms of alternative locomotion (e.g. walking, trotting, swimming) and in-phase locomotion (galloping, jumping) result from the interaction of 'programmes' for the coordination of (1) the homologous limbs (pair of hindlimbs or pair of forelimbs) and (2) the homolateral limbs (hind- and forelimb of the same side of the body). The movements of the homologous pairs of limbs are coupled out of phase in alternate locomotion and approximately in phase in the phase form of locomotion. The movements of the homolateral pairs of limbs occur approximately out of phase in the trotting type of coupling and approximately in phase in the pacing type of coupling. Transitions between the different forms of coupling occur abruptly over 1 or 2 steps. Therefore, for each type of coupling (homologous or homolateral) there are two distinct forms or 'programmes' of movement. The hypothesis is advanced that (a) all the characteristic patterns of locomotion in the cat result from different combinations of these 'programmes' of homologous and homolateral limb coupling; (b) the 'programmes' are mutually self reinforcing in the gaits in which the coordination of the movements of the 4 limbs is bilaterally symmetrical; (c) the 'programmes' act in competition in certain gaits which are not bilaterally symmetrical giving rise at times to a changing gait pattern, and (d) the temporary dominance of one 'programme' or another can determine the gait of the particular step.