Bilateral lesions placed in the pontiene tegmentum resulted in episodes of paradoxical sleep in which the characteristic atonia of that sleep stage was absent in six cats. Following each period of synchronized sleep, in which the degree of muscle tone of the dorsal cervical muscles gradually diminished, cats with such lesions would slowly raise their heads, move their limbs at all joints, make several attempts to rise and eventually leap violently. During such episodes they were unresponsive to strong lights, touching and mild pinching. Only sound would arouse them. This behavior appeared as early as the 2nd postoperative day, the 1st day of recording. Such episodes supplanted normal paradoxical sleep with atonia and lasted unchanged for as long as 6 months in one cat until it was killed while still in good health. Complete recovery of atonia was observed in one cat after 3 weeks. Either no recovery or else eventual recovery to excessively active periods of paradoxical sleep while remaining recumbent characterized the sleep of the other four. The conclusion drawn from these experiments and from a review of the literature is that the hypotheses stating that the locus coeruleus or other isolated nuclei of the pons are specifically concerned with the initiation of paradoxical sleep are not clearly supported by available evidence.