The sensory control of lamprey dorsal fin motoneurons was studied by using paired intracellular recordings combined with a morphological analysis. Dorsal cells innervating the skin of the dorsal fin and fin motoneurons were retrogradely labeled by injecting fluoresceincoupled dextran amines into the dorsal fin. Labeled motoneurons and dorsal cells showed close appositions, suggesting that the dorsal cells innervating the fin region make monosynaptic connections with fin motoneurons. By using conventional electrophysiological criteria, monosynaptic excitatory connections were found between fin dorsal cells and fin motoneurons. In addition, Lucifer yellow injection followed by confocal three-dimensional (3-D) reconstructions of monosynaptically connected pairs, revealed close apposition between dorsal cell axons and the distal dendrites of fin motoneurons. Each fin motoneuron received monosynaptic excitatory input from at least four different afferents. The amplitude of the monosynaptic excitatory postsynaptic potential (EPSP)s was reduced by administration of the N-methyl-D-aspartate (NMDA) receptor antagonist DL,2 amino-5-phosphovaleric acid (APV). Sensory stimulation could also elicit di- or oligosynaptic inhibitory postsynaptic potential (IPSP)s, which were blocked by the glycine antagonist strychnine, resulting in the appearance of large monosynaptic EPSPs, which could induce action potentials.