Substance P (1-7) is a fragment that gives depressor and bradycardic effects when applied to the nucleus tractus solitarius. A tritium-labeled aminoterminal heptapeptide of SP, 3H- SP(1-7), was synthesized, purified, and used to characterize the binding of a variety of fragments of SP and opioids in the mouse brain and spinal cord membranes. The characteristics of SP (1-7) binding sites are consistent with those expected for an SP N-terminal receptor.