Capsaicin and resiniferatoxin are natural products which act specifically on a subset of primary afferent sensory neurons to open a novel cation-selective ion channel in the plasma membrane. These sensory neurons are involved in nociception, and so, these agents are targets for the design of a novel class of analgesics. Although synthetic agonists at the capsaicin receptor have been described previously, competitive antagonists at this receptor would be interesting and novel pharmacological agents. Structure-activity relationships for capsaicin agonists have previously been rationalized, by ourselves and others, by dividing the capsaicin molecule into three regions--the A (aromatic ring)-, B (amide bond)-, and C (hydrophobic side chain)-regions. In this study, the effects on biological activity of conformational constraint of the A-region with respect to the B-region are discussed. Conformational constraint was achieved by the introduction of saturated ring systems of different sizes. The resulting compounds provided agonists of comparable potency to unconstrained analogues as well as a moderately potent antagonist, capsazepine. This compound is the first competitive antagonist of capsaicin and resiniferatoxin to be described and is active in various systems, in vitro and in vivo. It has recently attracted considerable interest as a tool for dissecting the mechanisms by which capsaicin analogues evoke their effects. NMR spectroscopy and X-ray crystallography experiments, as well as molecular modeling techniques, were used to study the conformational behavior of a representative constrained agonist and antagonist. The conformation of the saturated ring contraint in the two cases was found to differ markedly, dramatically affecting the relative disposition of the A-ring and B-region pharmacophores. In agonist structures, the A- and B-regions were virtually coplanar in contrast to those in the antagonist, in which they were approximately orthogonal. A rationale for agonist and antagonist activity at the capsaicin receptor is proposed, based on the consideration of these conformational differences.