The anatomical pathways from the parvocellular reticular formation and medulla oblongata to the mesencephalic trigeminal nucleus (Mes V) were investigated in the rat by means of the anterograde tracer Phaseolus vulgaris leukoagglutinin. P. vulgaris was injected into the rostral ventromedial medulla, rostral and caudal ventrolateral medulla, the parvocellular reticular formation and surrounding regions. Rostral medullary regions and the parvocellular reticular formation projected to Mes V although the bilaterality, extent and density of the projections originating in these reticular structures showed clear variations. In experiments in which dorsolateral reticular areas were involved, we found the projections to the bilateral trigeminal nuclei to be most prominent. More caudal areas, including the spinal trigeminal nucleus caudalis, showed no projections to MesV.The present experiments show, for the first time, the existence of ipsilateral and crossed projections from the reticular formation to the MesV.These pathways are possibly involved in direct modulation of trigeminal reflexes.