Dopamine β-hydroxylase (DBH; EC 1.14.17.1) is a copper-containing mixed-function oxidase that catalyzes the penultimate step in the catecholamine biosynthetic pathway, converting dopamine (DA) to norepinephrine (NE). A selective DBH inhibitor is expected to lower NE levels to blunt noradrenergic drive while elevating DA levels to exert beneficial peripheral dopamine receptor activation, with a low risk of tachyphylaxis due to limited DBH upregulation. Compounds capable of binding to both the phenethylamine substrate site and the active site copper atoms of DBH are hypothesized to be potent inhibitors. We found that certain 1-benzylimidazole-2-thiols inhibit DBH through such dual binding. Here we report the synthesis and pharmacological evaluation of several of these compounds (3a-c), which are extraordinarily potent inhibitors of DBH. Compound 3b binds DBH approximately 10^6-fold more tightly than the natural substrates dopamine and tyramine. In vitro, 3a-c exhibit low Ki values (5.7-55 nM) against homogeneous bovine DBH. In vivo, these compounds are orally active in spontaneously hypertensive rats (SHR), causing significant increases in the DA/NE ratio in the mesenteric artery and reductions in mean arterial blood pressure. Compound 3c is more potent than the standard DBH inhibitor fusaric acid, showing a dose-dependent increase in the DA/NE ratio, rapid onset of antihypertensive activity, and no tachyphylaxis even after 8-10 weeks of chronic administration. The nature of the substituent at position 1 of the imidazole-2-thiol moiety is critical for inhibitory activity, as 3a-c are 100-1000-fold more potent than previously described simple 1-alkylimidazole-2-thiols. Eight new dermorphin tetrapeptides bearing D-methionine S-oxide at position 2 (X-Tyr-D-MetO-Phe-aa-Y; X = H or H2N=C(NH); aa = Gly, 2-aminoethanol, or sarcosine; Y = NH2 or NH-) were synthesized and evaluated for opioid activity. These peptides show dose-related, naloxone-reversible opioid effects both in vitro and in vivo. H-Tyr-D-MetO-Phe-Gly-NH2 (I) demonstrates higher central opioid activity than natural dermorphin (H-Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2), with an IC50 of 13.6 nM in guinea pig ileum and ED50 values of 1.97 pmol/mouse (intracerebroventricular, icv) and 0.65 pmol/kg (subcutaneous, sc) in the mouse tail-flick test. Compared to morphine, I is approximately 1500-fold more potent when administered icv and 17-fold more potent when given sc. Previous studies on 'small dermorphins' (tetrapeptides X-Tyr-D-Ala-Phe-aa-Y) showed potent analgesic activity after icv injection but weak activity after sc administration. It has been reported that [D-MetO2]-containing enkephalins and dermorphin peptides retain high potency in vitro and/or after sc administration, suggesting a key role for the D-MetO2 residue. Accordingly, we report the synthesis and opioid activity of these new dermorphin tetrapeptides with D-methionine S-oxide at position 2.