In anesthetized cats, phrenic motoneurons were intracellularly labeled with HRP. Immunohistochemistry was used to localize serotonin-like immunoreactivity that was present in numerous boutons and nerve fibers within the ventral horn of the C5 spinal segment. Immunoreactive boutons were frequently found in apposition to phrenic motoneurons, but these close contacts were more common on the dendrites than the cell body. At the electron microscope level, serotonin-immunoreactive boutons were found to make synapses with well-defined postsynaptic densities on proximal and distal dendrites of phrenic motoneurons. These results suggest that serotonin-containing neurons may directly affect the excitability of phrenic motoneurons, mainly through an input onto their extensive dendritic trees.