A new series of cyanoamidines, N-arylalkyl-N'-cyano-3-pyridinecarboxyamidines was synthesized and evaluated for inhibitory effects on 40 mM KCl-induced contraction and norepinephrine (NE)-induced contraction of rat aorta strips. The N-phenethyl cyanoamidine 4c showed potent vasodilatory action. Further in vitro screening program using 4c as a lead compound resulted in the discovery of highly potent N-[2-(2-chlorophenyl)ethyl]-N'-cyano-3-pyridinecarboxamidine (5j). Compound 5j induced the greatest increase in ⁸⁶Rb⁺ efflux among cyanoamidine series. Subsequent modification of the pyridine ring of 5j was performed with evaluation for intravenous and oral antihypertensive activities. Introduction of an amino group at the 5-position of the pyridine ring furnished the new potassium channel opener, 5-amino-N-[2-(2-chlorophenyl)ethyl]-N'-cyano-3-pyridinecarboxamidine (9e; KRN4884), which showed highly potent antihypertensive activity and a long duration of antihypertensive action after oral administration. KRN4884 is under further development as an antihypertensive agent.