This review documents my research for the past 45 years in peptide chemistry. Initially, in order to study the structure-activity relationships of active center of α- and β-melanocyte stimulating hormones (H-His-Phe-Arg-Trp-Gly-OH), we employed D-amino acids. That approach yielded first published report in 1965 of antagonists containing D-amino acids. Monkey β-melanocyte stimulating hormone (β-MSH), an 18 amino acid peptide stimulated pigment cells. We synthesized β-MSH and fragments thereof, and studied in detail structure-activity relationships. A major and valuable result revealed that the C-terminal pentadecapeptide of β-MSH exhibited higher MSH activity than the parent hormone providing a new question; namely, what was the role of the N-terminal tripeptide? In order to identify the novel enzyme, spleen fibrinolytic proteinase (SFP), I developed a specific chromogenic substrate, Suc-Ala-Tyr-Leu-Val-pNA, and a specific inhibitor, Suc-Tyr-D-Leu-D-Val-pNA, once again employing my D-amino acid strategy. SFP was purified by affinity chromatography using Suc-Tyr-D-Leu-D-Val-pNA as the bound ligand. The success of this approach provided me the incentive to develop a variety of potential drugs. Thus, I prepared a specific plasmin inhibitor (YO-2) and a plasma kallikrein inhibitor (PKSI-527). Next, my research developed novel opioid receptor specific opioid agonists and antagonists based on 2′,6′-dimethyl-L-tyrosine (Dmt) dimers coupled with unique pyrazinone ring as a spacer. They exhibited potent oral antinociceptive activity acting through the μ-opioid receptor. Potent μ-receptor agonists (H-Dmt-Pro-Phe/Trp- Phe-NH₂) were transformed into highly selective μ-receptor antagonists (N-allyl-Dmt-Pro-Phe/Trp-Phe-NH₂), which reversed ethanol-induced increases in GABAergic neurotransmission, suggesting the possibility that they may emerge as candidates for the treatment of ethanol addiction.