PURPOSE: To provide a novel 4-aryl-thiazole or imidazole deriv. which has an effect of increasing the sensitivity of cardiac myofibrils to calcium and is useful for the treatment of cardiac failure and asthma or its medicinally acceptable salt.

CONSTITUTION: The 4-aryl-thiazole or imidazole deriv. of formula I [X is O, NOH, Y is S, NR3 (R3 is H, alkyl); Ar is phenyl, naphthyl, tetrahydronaphthyl, biphenyl; R is H, hydroxy, alkyl, alkoxy, thioalkyl, cycloalkyl, halogen, CF3, NO2, etc.] or medicinally acceptable salt(exclusive of 4-(4-chlorophenyl)- thiazole-2-carboxyamide), for example, 4-(3-chloro-4,5-dimethoxyphenyl)-N- hydroxy-thiazole-2-carboxyimidazole. The compd. of the formula I is obtd. by condensing the compd. of formula II with ammonia and converting the resulted compd. in which X is O, to the compd. in which X is NOH by desire.