The light microscopic and ultrastructural distribution of somatostatin immunoreactivity has been studied in laminae I-III of the rat cervical spinal cord by means of a bi-specific anti-somatostatin/anti-horseradish peroxidase monoclonal antibody. Immunoreactivity was demonstrated in small rostro-caudally oriented nerve cells of ventral lamina II. Somatostatin-immunoreactive axonal varicosities contained round, agranular, synaptic vesicles and some large granular vesicles. These varicosities established either symmetric or asymmetric synaptic contacts with dendrites, presynaptic dendrites or cell bodies. In the middle third of lamina II, a small number of somatostatin-immunoreactive varicosities were the central elements of type I synaptic glomeruli. Immunoreactivity for somatostatin was also detected in dendritic profiles of laminae II-III. Some of these dendrites were part of synaptic glomeruli, and a small number of them were presynaptic dendrites. The latter were sometimes presynaptic to the central glomerular bouton. The results favor a participation of somatostatin-containing spinal interneurons in the modulation of sensory information.