6-(p-Acylaminophenyl)-4,5-dihydropyridaz-3-ones of the formula I ##STR1## where R.sup.1 is hydrogen or alkyl of 1 to 3 carbon atoms and R.sup.2, if R.sup.1 is hydrogen, is halogen-substituted alkyl of 3 to 6 carbon atoms or .beta.-haloethyl or, if R.sup.1 is alkyl of 1 to 3 carbon atoms, is halogen-substituted alkyl of 1 to 6 carbon atoms, their manufacture, and therapeutic agents which contain, as the active ingredient, a compound of the formula I, where R.sup.2 may also be halomethyl or .alpha.-haloethyl if R.sup.1 is hydrogen. These compounds may be used as anti-hypertensive agents and for the prophylaxis and therapy of thromboembolic disorders.