Abstract: 1-(3-Mercapto-2-methylpropanoyl)prolyl amino acid derivatives of the formula ##STR1## wherein R represents a hydrogen atom, a lower alkyl group, a phenyl-lower alkyl group or a substituted phenyl-lower alkyl group, R.sub.1 represents a hydrogen atom, R.sub.4 CO--, R.sub.5 S--, or ##STR2## R.sub.2 represents a hydrogen atom or a lower alkyl group, R.sub.3 represents a hydrogen atom, a phenyl group, a lower alkyl group, or a substituted lower alkyl group, or R.sub.2 and R.sub.3 form a heterocyclic ring together with the nitrogen and carbon atoms to which they are respectively bonded; salts of these compounds; compositions comprising these compounds or salts; and processes for preparing these compounds or salts. These compounds and salts have excellent antihypertensive activity.

Abstract: A compound of the formula ##STR1## wherein R.sub.1 represents a hydrogen atom or a lower alkyl group, and R.sub.2 represents a hydrogen or halogen atom or a trifluoromethyl group,And its pharmaceutically acceptable salt. The compound is prepared by reacting the corresponding 1,2,4-benzothiadiazine 1,1-dioxide with a piperazine compound or alkylating 1,2,4-benzothiadiazine 1,1-dioxide whose corresponding 3-position is substituted by a piperazinyl group. The above compound has superior antihypertensive activity separated from diuretic, antidiuretic and hyperglycemic activities, and is free from side-effects.