Thyrotropin-releasing hormone (TRH) produced a depolarization in lumbar motoneurons of neonatal rats. The depolarization by TRH persisted after extracellular Ca2+ was replaced by Mg2+ or Mn2+, indicating its direct action upon motoneurons. Stimulation of the ventral descending tract at the lower thoracic segment evoked slow excitatory postsynaptic potentials (e.p.s.ps) lasting 20-30 s in every motoneuron. Both the TRH-induced depolarization and descending slow e.p.s.p. were accompanied by a decrease in input conductance of motoneurons. When the membrane potential of the motoneuron was shifted, both the TRH-induced depolarization and slow e.p.s.p. became larger in amplitude during depolarization and smaller during hyperpolarization. However, they could not be reversed in polarity by hyperpolarization. During the depolarization of motoneuron produced by TRH application, the slow e.p.s.p. was markedly reduced in amplitude, suggesting the involvement of identical ionic mechanisms in the two responses. After incubation of the isolated spinal cord with antisera to TRH, the depolarizing response produced by TRH as well as the descending slow e.p.s.p. was greatly diminished. In contrast, monosynaptic reflexes evoked by dorsal root stimulation remained unchanged under this condition. These results suggest that TRH serves as a neurotransmitter mediating the descending slow e.p.s.p. in motoneurons.