In an effort to develop antihypertensive agents with peripheral vasodilator activity, a series of 40 novel 3-hydrazino-5-phenyl-1,2,4-triazines (II) were synthesized and evaluated in the spontaneously hypertensive rat assay (SHR assay). Based on the performance of the structurally related standard, hydralazine (I), 15 triazines were active. Thirteen of these hypotensive triazines possessed LD50 values in the mouse greater than I (LD50 = 100 mg/kg); only one active triazine had an LD50 value greater than 300 mg/kg (11d). Four asymmetric triazines had moderate antihypertensive activity and LD50 values greater than 300 mg/kg (6b, 7c, 8f, and 9g). Based on the relationship between toxicity and antihypertensive activity, three triazines (8f, 9g, and 11d) were chosen for dose-responses studies in the SHR assay. None were as efficacious as I, but all three were less toxic, resulting in similar therapeutic indices relative to I.