Quinolone derivatives having a quinolone structure: ##STR1## where Yo is O or S are disclosed, which are useful as cardiotonic agents. Typical examples of the quinolone derivatives include: 6,7-dimethoxy-4 (1H) quinolone (compound 6) and 5-hydroxy-6-methoxy-4(1H) quinolone (compound 1) as typical compounds of the formulae[I] and [I'], respectively, shown in the specification.