The activity of the rat hindlimb during treadmill stepping was studied in normal adult rats and in rats in which cerebellar development was interfered with by early-postnatal focal X-irradiation. Based on cinematographic and electromyographic data from over 100 step cycles in 15 normal rats, correlations were made between joint angles and muscle activity to obtain a detailed picture of the locomotor pattern of the rat hindlimb. It was possible to relate most of the features of limb movement to activity in one or more of six primary flexors and extensors of the hindlimb. Compared with available data in the cat or dog, the joint angle curves were similar in shape except that the knee joint angle was usually greater at foot contact than at lift-off, while in cats and dogs the reverse is usually the case. This difference is due to a more crouched stepping posture in the rat in which the leg is not extended as far backward as in the cat or dog. It was also noticed that there was more side-to-side bowing of the spine in the rat during stepping. Finally, in rats there was no correlate to the stance phase burst seen in the semitendinosus in cats. In rats with cerebellar X-irradiation there was little effect on the stepping cycle if the animal's equilibrium was maintained externally, either by a supporting harness or by immersion in water (swimming). However, when stepping without external support, animals were unable to adequately compensate for perturbations in equilibrium, resulting in an ataxic gait. This problem was compound by the presence of high-frequency (18 Hz) tremor which, by producing hyper- or hypotonia during critical periods of stepping such as foot placement or during corrective reflex movements, was a major disturbing force to the animal's equilibrium.