Hydrazinopyridiazine derivatives of the formula: ##STR1## wherein A is alkylene, wherein either R.sup.1 is hydrogen and R.sup.2 is hydrogen, alkoxycarbonyl or aralkoxycarbonyl, or R.sup.1 and R.sup.2 together form alkylidene, aralkylidene, alkoxycarbonylalkylidene or aralkoxycarbonylalkylidene, wherein either R.sup.3 and R.sup.4, which may be the same or different, each is hydrogen, halogen, hydroxy, amino, nitro, trifluoromethyl, carbamoyl, cyano, alkyl, hydroxyalkyl, cycloalkyl, alkenyl, alkynyl, alkoxy, alkylthio, cycloalkoxy, alkenyloxy, alkynyloxy, alkanoyl, aryl, aryloxy or dialkylamino, or R.sup.3 and R.sup.4 together with the adjacent benzene ring form indanyl, 5,6,7,8-tetrahydronaphthyl, 5-oxo-5,6,7,8-tetrahydronaphthyl, indenyl, 5,8-dihydronaphthyl or naphthyl, wherein either R.sup.5 and R.sup.6, which may be the same or different, each is hydrogen or alkyl, or R.sup.5 and R.sup.6 together with the adjacent pyridazine ring form phthalazinyl, and wherein X is oxygen or --NHCO--, --NHCO--A.sup.1 -- or --NHCO--A.sup.1 --O-- wherein A.sup.1 is alkylene, acid-addition salts thereof, processes for their manufacture and pharmaceutical composition containing them. The compounds possess antihypertensive activity.