Abstract: Twenty-three derivatives of 5-substituted-4-methyl-1,3-dihydro-2 H-imidazol-2-one were designed and synthesized for searching of more potent and less toxic cardiotonic agents. The key intermediate, 4-methyl-1,3-dihydro-2H-imidazol-2-one(Ⅳ), was prepared by a method of one-step oxidation of β-hydroxy propylaminc, a new synthesis of aminoacetone.Preliminary pharmacologic tests on isolated male guinca pig atria showed that eight of the synthetic compounds (Ⅱ₁, Ⅱ₂, Ⅱ₄, Ⅲ₄, Ⅲ₆, Ⅲ₇, Ⅲ₈ and 1) exhibited significant positive inotropic activities. The maximum inotropic effects of four of them (Ⅱ₁, Ⅱ₂, Ⅱ₄ and Ⅲ₄) were higher than those of the known cardiotonic agent, RMI 82,249(1). It is interesting that Ⅱ₄ showed both positive inotropic and negative chronotropic effects significantly.