Using indirect immunofluorescence, neuropeptide Y Y1 receptor (Y1 receptor)-like immunoreactivity (LI) was localized close to the plasmalemma of small neurons in lumbar dorsal root ganglia (DRGs) and neurons in the inner lamina II of the lumbar spinal cord of the rat. Using confocal microscopy, colocalization of Y1 receptor-LI and transferrin receptor-LI, a marker for endosomes and coated vesicles, was observed in dot-like structures along the plasmalemma. Under the electron microscope, Y1 receptor-LI was localized in coated vesicles and endosomes, in the membrane of tubular cisternae, sometimes connected to multivesicular bodies, and in the plasmalemma. These complex distribution patterns may reflect receptor turnover and internalization processes. In the lamina II of the spinal dorsal horn, Y1 receptor-LI was localized in the plasmalemma of neurons without any apparent association with paramembrane structures, as described above for the DRG neurons. Many dendrites were Y1 receptor-positive, and some of them made synaptic contacts with unstained axonal terminals. In general, Y1 receptor-LI was localized in the membrane outside the postsynaptic density. Double-immunofluorescence staining showed that most Y1 receptor-immunoreactive neurons in lamina II contained somatostatin-LI. Both in DRG and dorsal horn neurons, the Y1 receptor thus seems to represent a postjunctional/postsynaptic receptor.