The electromyographic (EMG) activity of the lumbar back muscles (multifidus, longissimus and iliocostalis) was investigated during treadmill locomotion in acute high decerebrate and in low spinal cats. During alternate stepping (0.7-2.0 m X s-1) in high decerebrate cats, the back muscles have two bursts of activity per step cycle. On the average these EMG bursts last about 170 ms and start some 25 ms before the onset of each vastus lateralis (VL). The two bursts in any one back muscle may have a different duration, the shortest burst on one side coinciding with the longest burst of the homologous contralateral muscle. There is often an overall asymmetry in the discharge wherein the bursts of activity in both ipsi- and contralateral muscles are longer at the onset of one of the VLs. Correlation analyses of several timing parameters of the bursts as a function of walking speeds were made. Different patterns of correlation were identified and it was found that, in most cases, the end of the bursts (with respect to the onset of VL activity) was best correlated with the speed of walking. During gallop, the back muscles activity is a single burst of about 200 ms duration which starts some 75 ms before the onset of VL. In low spinal cats walking after an injection of clonidine, the double burst pattern of EMG activation may be present if there is adequate weight support. However, when the animal steps with the hindlimbs extended and with insufficient weight support, these muscles have a tonic activity uncorrelated with the rhythmic activity of hindlimb muscles.