Eight cyclic analogues of Substance P were made in order to investigate the conformation of the C-terminal end of the peptide. These analogues were designed to test three literature models describing the active conformation of substance P. Although the potencies of the analogues were low (in the micromolar range), our results support Cotrait's and Hospital's model (1986). Several substance P antagonists were synthesized. These compounds did not demonstrate agonistic activity nor anatagonistic activity. The tryptophan side chain is contributing to the antagonistic activity of these analogues, and not just the chirality of the α-carbon. Highly potent and selective photoaffinity ligands of H-Tyr-D-Pen-Gly-Phe-D-Pen-OH (DPDPE) and D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH₂ (CTP) were synthesized. These compounds will be useful in the isolation of δ and μ opioid receptors. Several new amino acids designed and synthesized to contain both the natural amino acid side chain and a thiol group which can be used to make disulfide constraints. The racemic amino acids made were as follows: (1) 2-amino-4-methyl-2- [(p-methylbenzyl)thiomethyl] pentanoic acid; (2) 2-amino-2- [(p-methylbenzyl)thiomethyl] -3-phenylpropanoic acid; (3) 2-amino-e- [(p-methylbenzyl)thio] pentanoic acid; and (4) 2-amino-3- [(p-methylbenzyl)-thio] -3-phenyl-pentanoic acid. These amino acids will be useful in the conformational restriction of peptides. To investigate the δ-opioid receptor conformation proposed for DPDPE by Hruby et al. (1988) and the μ-opioid receptor conformation proposed for Tyr-c [Abu₂,Gly,Phe,Leu] by Mierke et al. (1988), constrained phenylalanine amino acids were incorporated into H-Try-D-Pen-Gly-Phe-D-Pen-OH (DPDPE) in the four position. Our results indicate that these models are correct. And in an investigation into the physical-chemical properties of the delta opioid receptor, our results suggest that the δ receptor topochemical site for the Phe⁴ residue contains a partial positive charge on its surface and has specific steric requirements.