Reactions of-methyl-2(3H)-thiazolones (5) with various N-alkoxycarbonyl pyridinium salts (6a-f) led to (N-alkoxycarbonyl dihydropyridyl)thiazolones (7a-f), oxidation of which yielded a new class of 5-pyridylthiazolones (8a-f). These reactions were applied to the synthesis of other azaarylthiazolones. Some of these azaarylthiazolones, particualrly 5-(4-pyridyl)thiazolones (8b, c) and 5-(4-quinolyl)thiazolones (14a, b), showed positive inotropic activity with little chronotropic effect on guinea pig left atria.