The distribution, morphology, and central projections of the mesencephalic trigeminal neurons in the frog Rana ridibunda were studied with tracing techniques. Retrograde tracing with horseradish peroxidase (HRP) or the fluorescent tracer Fluorogold, and anterograde tracing by means of Phaseolus vulgaris leucoagglutinin, the fluorescent dye Dil, and HRP were used. The mesencephalic trigeminal nucleus (MesV) of Rana ridibunda is formed by a population of 100 to 125 unipolar or multipolar cells that are scattered on both sides of the rostral mesencephalic tectum. Subpopulation of Mes V cells were labeled after tracer application to ophthalmic, maxillary, and mandibular trigeminal branches, separately. Differences in the morphology and distribution of cells in these experiments were not evident but the number of neurons labeled via the maxillary nerve was always the highest. Mes V cells have a single central branch that courses caudally in the brainstem. At different levels, it bifurcates into a peripheral branch, which leaves the brain via the trigeminal root, and a descending branch, which terminates in a region in, or close to, the trigeminal motor nucleus and in a supratrigeminal location. The lack of a distinct somatotopy in the distribution of Mes V cells and the lack of projections caudal to the trigeminal motor nucleus as revealed in this study with a wide variety of tracers are in striking contrast to previous data provided for other amphibians. © 1993 Wiley-Liss, Inc.