Midbrain neuronal activity that correlated with the initiation of locomotion produced by hypothalamic stimulation was studied. Locomotion was elicited by electrical stimulation in the perifornical hypothalamus of 59 rats anesthetized with Nembutal. The first hindlimb extension indexed stepping onset. Single and multiple neurons were recorded ipsilateral to the stimulation site at 2230 sites in the anterior and posterior midbrain. To classify responses, activity patterns averaged around stimulation onset and around the extension onset were examined. Responses with specific correlations to extension onset were Type I; responses not specifically related to the extension onset were Type II. In the anterior midbrain, 6% of sites were Type I and 8% were Type II. The larger Type I responses were frequent in the anterior tegmentum near the central gray. The relative frequency of Type I patterns in the posterior ventrolateral tegmentum was similar. Other regions showed relatively more Type II responses; they included the ventral tegmental area, and the regions near the superior cerebellar peduncle and the posterior central gray. Regional population profiles showed that during the initiation of locomotion, neurons in the posterior peribrachial region responded early and neurons in the anterior dorsal and the posterior ventrolateral tegmentum responded later. The initiation-related activity of Type I neurons in the anterior and posterior midbrain tegmentum suggest that they warrant further study for a role in locomotor initiation.