During walking, a change in speed is accomplished by varying the duration of the stance phase, while the swing phase remains relatively invariant. To determine if this asymmetry in the control of locomotor cycles is an inherent property of the spinal central pattern generator (CPG), we recorded episodes of fictive locomotion in decerebrate cats with or without a complete spinal transection (acute or chronic). During fictive locomotion, stance and swing phases typically correspond to extension and flexion phases, respectively. The extension and flexion phases were determined by measuring the duration of extensor and flexor bursts, respectively. In the vast majority of locomotor episodes, cycle period varied more with the extension phase. This was found without phasic sensory feedback, supraspinal structures, pharmacology or sustained stimulation. We conclude that the control of walking speed is governed by an asymmetry within the organization of the spinal CPG, which can be modified by extraneous factors.