Using the retrograde tracer technique with horseradish peroxidase (HRP), attempts were made to determine the cells of origin and the descending pathway of the tegmentoreticular projections in order to give an anatomical substrate for the physiological phenomenon of the postural atonia observed during paradoxical sleep (PS) in the cat. The HRP was injected into various parts of the pontomedullary reticular formation (RF) including the caudal raphe nuclei, nucleus (n.) reticularis gigantocellularis (Gc), n. reticularis magnocellularis (Mc), and other pontomedullary structures adjacent to the Mc. The results indicated that the HRP injection into the Mc, particularly its caudal and lateral two-thirds, resulted in specific labeling of cells located in an area just medial to the LCa together with those in the most medial part of the LCa. Bilateral lesions of these pontine structures have been reported to suppress the atonia otherwise observed during PS in the normal cat. In addition to the HRP labeled cells, we have also observed HRP filled fiber bundles directed to labeled cells in the medial part of the LCa and immediately adjacent tegmental RF area. The same course of HRP labeled fiber bundles was also observed together with HRP labeled cells in the Mc after HRP injections into the medial part of the LCa area, indicating the existence of an interconnection between the LCa area and the Mc. The location of the tegmentoreticular pathway corresponded to that of the lesions effective to suppress the muscular atonia during PS. HRP injections into the caudal medullar caudal to the Mc, on the other hand, resulted in no or almost no HRP labeled cells in the area medial to the LCa, in spite of the presence of HRP containing neurons in other parts of the pontomedullary RF areas, showing that the tegmentoreticular projections as described above terminate almost exclusively in the Mc.