Recently we reported significant modulation of climbing fiber discharge in cerebellar Purkinje cells during normal and perturbed locomotion in the decerebrate cat walking on a treadmill. In this study covariation of simple spike activity and step cycle behavior with complex spike discharge were studied in decerebrate cats. Purkinje cell simple and complex spike discharge was recorded extracellularly in the intermediate region of lobules IV and V. Forelimb triceps and biceps electromyographic activity and displacement were monitored during the step cycle. A series of analyses were carried out to determine the temporal relationship between the complex spike discharge and forelimb step cycle, electromyographic activity and simple spike discharge. In this paper only the complex spike discharge associated with the onset of locomotion was evaluated. Using a sorting technique the amplitude of the forelimb step cycle and the associated triceps and biceps electromyographic activity covaried with complex spike discharge. For the majority of cells the alterations in the step cycle followed or occurred with the increase in complex spike discharge. However, in some cells the step cycle modifications preceded the increase in climbing fiber allerent activity. Another series of analyses employing an alignment technique demonstrated that a short term increase in simple spike discharge followed and was tightly coupled to the complex spike discharge. Additionally in most Purkinje cells an "oscillation" of simple spike activity which followed the complex spike discharge was uncovered. These observations support an important role for the climbing liber allerent system in ongoing motor behavior. The results are consistent with the speculation that increased climbing liber allerent input alters cerebellar cortical output which in turn can alter the ongoing motor behavior.