The ultrastructural localization of substance P-like immunoreactivity (SPLI) in lamina I (marginal zone) and lamina IIO (outer substantia gelatinosa) of the dorsal horn of the macaque monkey was examined by the indirect antibody peroxidase-antiperoxidase method. SPLI was found in small unmyelinated and finely myelinated axons and a variety of terminal types. The majority of SPLI terminals contained a few to many large granular vesicles (mean diameter 90 nm) in addition to a population of small clear round vesicles. A very few terminals contained mainly small round vesicles. SPLI terminals were presynaptic in axosomatic, axodendritic and axospinous contacts forming, in all but the axosomatic junctions, asymmetrical synapses. Some axosomatic junctions were symmetrical. SPLI terminals also formed the center of glomeruli with unlabeled dendrites and dendritic spines; some of the unlabeled dendrites contained a few small scattered vesicles and large dense-core vesicles. In more complex formations 2 to 4 SPLI terminals were associated with one another and linked by desmosomal contacts. The individual terminals in the complexes or 'congregate terminals' were simple large granular vesicle containing terminals (LGV), LGV-central terminals of associated glomeruli, or terminals containing mainly small round vesicles. In the apical region of lamina I an unlabeled terminal was found occasionally in contact with an SPLI terminal, which in turn synapsed onto a dendrite. These contacts have some synaptic characteristics and the SPLI terminal was possibly postsynaptic. Most of the types of SPLI terminals resemble closely terminal types shown to be of primary afferent origin. These terminals which make direct contact with dorsal horn dendrites may be the morphological substrate for postsynaptic excitation of dorsal horn neurons by substance P. The contacts of unlabeled terminals with SPLI terminals may represent a morphological substrate by which other neurochemical substances such as enkephalin or serotonin may modulate the substance P effects on dorsal horn neuronal activity.