An agent for the prophylaxis or treatment of TXA.sub.2 -mediated diseases, particularly, a TXA.sub.2 synthetase inhibitor, which comprises a pyridazinone compound of the formula (I) ##STR1## wherein each symbol is as defined in the specification, or a pharmacologically acceptable salt thereof as an active ingredient. The pyridazinone compound (I) and pharmacologically acceptable salts thereof used in the present invention have prophylactic and therapeutic activities against TXA.sub.2 -mediated diseases, particularly a TXA.sub.2 synthetase inhibitory action and are useful as an agent for the prophylaxis or treatment of TXA.sub.2 -mediated diseases, particularly as TXA.sub.2 synthetase inhibitors.