kappa-Opioid receptors and their endogenous ligand dynorphin A (Dyn A) are of tremendous interest because of their diversity of functions and potential for analgesics with fewer side effects. Compared to other opioid peptides such as the enkephalins, the study of the structure-activity relationships (SAR) of Dyn A have been limited, and the conformational preferences of the three key pharmacophoric groups, the N-terminal basic amine, Tyr1, and Phe4, in the "message" sequence of Dyn A are not yet clear. The main objectives of this research were to develop conformationally restricted analogs of Dyn A with high affinity and/or selectivity that can be used to explore the structural and conformational requirements for interaction of Dyn A with kappa-opioid receptors, and to explore the structure-activity relationships (SAR) for agonist vs. antagonist activity at kappa receptors. The central hypotheses of this research were that the spatial orientations of the three key pharmacophoric groups in the "message" sequence are important for the high affinity and activity of Dyn A for kappa-opioid receptors and that the spatial requirements of these groups for agonist activity are different from those for antagonist activity. Incorporation of conformational constraints in the "message" sequence will restrict and/or alter the relative position of these pharmacophoric groups thus effecting the affinity, selectivity and/or efficacy of the resulting peptides. Synthetic strategies were explored for the synthesis of side chain-to-side chain and novel head-to-side chain cyclic analogs of Dyn A. Modifications in the "message" sequence of cyclo[D-Asp2, Dap5]Dyn A-(1-11)NH2 resulted in peptides with high affinity for kappa receptors and high selectivity for kappa over delta receptors; however the selectivity of these peptides for kappa over mu receptors was low to modest. Substitution of Pro at position 3 of cyclo[D-Asp 2,Dap5]Dyn A-(1-11)NH2 resulted in a peptide with reduced efficacy for kappa receptors. The synthesis of novel head-to-side chain cyclic analogs of Dyn A with non-basic N-terminus resulted in peptides with nanomolar affinity and reduced efficacy for kappa-opioid receptors. The N-terminal cyclic analogs of Dyn A with basic N-terminus resulted in reduced affinity for opioid receptors. Results obtained during this research suggested that the spatial orientation of the key pharmacophoric groups is critical for opioid receptor affinity and activity of Dyn A analogs.