Experiments were performed on cats with chronic spinal lesions, in order to decide which systems either interlimb-propriospinal systems or direct descending systems, plays important roles for coordination of overground locomotion of the cat. The phase relationships between homolateral, crossed, bilateral forelimbs and hindlimbs were analysed during locomotion. For 27 cats a single hemisection was applied at the level of C1 or T12. In 19 cats, the spinal cord was firstly hemisected at around T12, then 5-183 days after, contralateral thoracic cord around T8 was hemisected (T-T; thoracic-thoracic bilateral serial hemisection). Therefore in this group, only crossed descending propriospinal systems were preserved and all other propriospinal systems and direct descending systems were severed. In 9 cats following spinal cord hemisection at around C1, at interval of 11-71 days the second hemisection was carried out at thoracic cord about T12 (C-T; cervico-thoracic bilateral serial hemisection). In this group homolateral and crossed propriospinal systems were left intact. In the single hemisection group, the phase relationships of all the six combinations of limbs were quite similar to normal group. Hence for the control of the phase relation during locomotion, only onesided neuronal circuits were needed. In both T-T and C-T group, polar diagram analysis revealed that the phase relationships were completely disturbed in any combination of the forelimbs and hindlimbs. It was concluded that the direct descending systems rather than propriospinal systems play important roles in control of phase relationship during locomotion.