A series of (substituted amino)-1,2,4-benzothiadiazine 1-oxides has been synthesized and most members of the series have been shown to have blood pressure lowering effects in normotensive rabbits and in spontaneously hypertensive rats. The most active member of the series was 3-[4-(2-furoyl)-1-piperazinyl]-6,7-dimethoxy-1-methyl-1H-1,2,4-benzothiadiazine 1-oxide hydrochloride. This compound in animal tests was equipotent to the known antihypertensive Prazosin.