The present study was carried out on rat heart to in situ determine the myocardial toxicity of moniliformin, a synthesized compound, originally isolated from mouldy corn and soil samples in the Keshan disease prevalent area in China. Perfusion of moniliformin 10(-7) mol/liter in isolated heart decreased myocardial contractile force by 52 +/- 17%. Intravenous injection of moniliformin at 1/6 and 1/4 LD50 markedly inhibited cardiac hemodynamic variables relative to myocardial contractile function. Particularly, it decreased +/- LV dP/dt max by 52 +/- 6%, and induced ventricular arrhythmia. These data indicate that moniliformin is toxic to mammalian heart and might be an important factor relative to Keshan disease.