The average EEG amplitude, average EMG amplitude and PGO spike rate per one minute epoch were measured for 3 days before and 21 days after pontine lesions in the cat. A trivariate computer graphics display of one baseline day's data (of 1350 epochs) revealed three major clusters of points that were automatically sorted by cluster analysis and corresponded to wakefulness, slow wave sleep and paradoxical sleep. Following combined medial-lateral caudal pontine lesions, the cluster of points that corresponded to the state of paradoxical sleep was absent. Two clusters were still evident and corresponded to wakefulness and slow wave sleep, which was characterized by higher than normal EMG amplitude and PGO spike rate. Whereas medial caudal pontine lesions alone did not reproduce these effects, lateral caudal pontine lesions did. These results suggest that cells and/or fibers located within the lateral tegmental field rather than those within the medial gigantocellular tegmental field of the pons are most important for the generation of the cluster of events that characterize paradoxical sleep.