Abstract The synthesis of several 4-and 5-alkoxy-substituted pyridazin-3 (2H)-ones with hypotensive and β-blocking activities by phase-transfer catalysis (PTC) starting from halopyridazin-3 (2H)-ones is described. The nucleophilic displacement reaction proceeds easily at the 5-position and remarkably slower at the 4-position. In the case of 4, 5- dihalopyridazin-3 (2H)-ones the PTC methods provide a high regioselectivity towards ...