Decreased right as well as left ventricular function can be associated with pulmonary hypertension (PH). Numerous investigations have examined cardiac function following induction of pulmonary hypertension with monocrotaline (MCT) assuming that MCT has no direct cardiac effect. We tested this assumption by examining left ventricular function and histology of isolated and perfused hearts from MCT-treated rats. Experiments were performed on 50 male Sprague-Dawley rats ( 348±6 (SD)). Thirty-seven rats received MCT (50 mg/kg s.c.) (MCT group) while the remainder did not (Control group). Three weeks later pulmonary artery pressure was assessed echocardiographically in 20 MCT and 8 Control rats. The hearts were then excised and perfused in the constant pressure Langendorff mode to determine peak left ventricular pressure (LVP), the peak instantaneous rate of pressure increase (+dP/dt_max) and decrease (-dP/dt_max), as well as the rate pressure product (RPP). Histological sections were subsequently examined. Pulmonary artery pressure was higher in the MCT treated group compared to the Control group (12.9±6 vs. 51±35.3 mmHg (P<0.01)). Left ventricular systolic function and diastolic relaxation were decreased in the MCT group compared to the Control group (+dP/dt_max 4178±388 vs. 2801±503 mmHg/s, LVP 115±11 vs. 83±14 mmHg, RPP 33688±1910 vs. 23541±3858 beats/min·mmHg, -dP/dt_max -3036±247 vs. -2091±389 mmHg/s; P<0.0001). The impairment of cardiac function was associated with myocarditis and coronary arteriolar medial thickening. Similarly depressed ventricular function and inflammatory infiltration was seen in 12 rats 7 days after MCT administration. Our findings appear unrelated to the degree of PH and indicate a direct cardio-toxic effect of MCT.