The coupling of movements of the hindlimbs and forelimbs has been analysed in intact cats stepping overground and on a treadmill and during swimming, and in decerebrate cats stepping on a treadmill, immersed in water('swimming') and stepping suspended in the air. In the different preparations, and under different types of locomotion, two basic patterns of coupling have been observed. Both concern the hindlimb and forelimb of the same side of the body. The first pattern is found in the pacing gait where flexion of the forelimb precedes extension of the hindlimb, measured at the elbow and knee, respectively. The second pattern is typically found in the trot where flexion of the forelimb follows extension of the hindlimb. In decerebrate cats both patterns of coupling remain after bilateral deafferentation of the hindlimbs. In the alternate form of locomotion these patterns of coupling occurs symmetrically on both sides. In the rotatory and transverse gallop (examples of the in-phase form of locomotion) the coupling is asymmetrical: on one side it is comparable to pacing (forelimb flexion precedes hindlimb extension), and on the other side to trotting (forelimb flexion follows extension). These basic patterns of interlimb coordination simplify considerably the problem of neural control of the limbs in locomotion. Obersations of EMGs during the alternative forms of locomotion show that in the pacing type of coupling the extensor EMGs of forelimb and hindlimb overlap, with the hindlimb leading the forelimb by about 10% of a step cycle, while in the trotting type of coupling the forelimb flexor EMGs overlap the hindlimb extensor EMGs, the forelimb flexors leading the hindlimb extensors by about 10% of a step cycle. During acceleration the transition between the two forms of EMG occurs within one or two step cycles, and at some intermediate velocities the EMG coupling springs back and forth between the two different forms. These results further support the hypotesis of two basic forms of interlimb coupling in which long propriospinal pathways probably play a role.