Structural non-synaptic appositions between serotonin (5-HT) nerve endings and neuropeptide Y (NPY)-containing neurons were demonstrated in the rat arcuate nucleus by means of a combined radioautographic and immunocytochemical detection of [3H]5-HT uptake sites and NPY-immunoreactivity. Such cellular relationships are proposed to constitute a morphological substrate for putative 5-HT/NPY interactions in neuroendocrine hypothalamus.