In freely moving cats, walking at speeds of 0.4-1.0 m/s, lesions of the lateral funiculi, performed at the low thoracic level, increased the hindlimb step cycle duration and changed the relationships between the stance and swing phase durations and the step cycle duration. The values of the slopes of linear regression lines relating the swing and the step durations were markedly increased, while those for the stance phase were decreased, compared to intact animals. Control dorsal column lesion produced no change in these parameters. The results suggest that pathways in the lateral funiculi play a substantial role in maintaining the proper structure of the step cycle.