The medullary projection from the mesencephalic trigeminal nucleus has been studied in cats where the wheat germ agglutinin-horseradish peroxidase complex has been used as a retrograde and anterograde tracer. All injections were made at the level of the caudal pole of the inferior olive and show that it is especially the lateral part of nucleus parvicellularis of the reticular formation which is the main area for termination of the fibres. In addition, it can not be excluded that the descending fibres also reach the medialmost part of the spinal trigeminal sensory nucleus (pars magnocellularis and the adjoining pars gelatinosa). All three cell types of the mesencephalic trigeminal nucleus are labelled, the large globular, the small globular and the polygonal cells. Some of these cells are only faintly labelled. The observations confirm previous studies of the intrinsic trigeminal connections, but show that when the wheat germ agglutinin-horseradish peroxidase complex is used as a tracer, the origin as well as the termination for the intrinsic fibres (also the commissural) can be studied in the same section. The use of this sensitive tracer also shows new details within the intrinsic connections. The findings are discussed with special reference to the recent observations by Ruggiero et al. (1982). These authors found that in rabbit and rat there is a more restricted termination for the descending mesencephalic trigeminal fibres. The discrepancies between the observations seem to indicate that there are species differences as regards the connections between the mesencephalic trigeminal nucleus and the nuclei in the lower part of the brain stem.