The present study was carried out on intact heart of rat to determine the myocardial toxicity of moniliformin (MLFN) in vitro and in vivo Perfusion of MLFN 0.1 mumol/L in isolated heart decreased myocardial contractile force by 52 +/- 17%, and the effect was not reversible after the infusion of MLFN had stopped. The intravenous LD50 of MLFN in mice was found to be 17.51 +/- 1.64 mg/kg. Intravenous injection of MLFN at 1/4 LD50 markedly inhibited cardiac hemodynamic variables relative to myocardial contractile function. Particularly, it decreased left ventricular pressure development (dP/dt max) by 52 +/- 6%, and induced ventricular arrhythmia. The results showed that MLFN severely damaged the heart function of healthy rats.