ObjectiveTo observe the pathological changes in heart and lung tissues in rats with pulmonary hypertension induced by monocrotaline. MethodsTwenty-four male Sprague-Dawley rats were randomly and equally divided into an experimental group and a control group. The rats in the experimental group were intraperitoneally injected with monocrotaline to induce pulmonary hypertension, and the rats in the control group were treated with saline. All rats were fed for 3 weeks, and the general situation were observed. Then the rats were sacrificed for measurement of mean pulmonary artery pressure (mPAP), right ventricular hypertrophy index [RV/(LV+S)], changes of myocardial cells and lung vascular, calculated density of middle membrane smooth muscle cells (SMC) in medium/small pulmonary arteries accompanied with bronchi and alveoli, media thickness of pulmonary artery (PAMT), the percentage of wall thickness with outer diameter (WT%), the percentage of wall area with total area (WA%), the average diameter of myocardial cells (AD), and myocardial nuclei density (MND). ResultsCompared with the control group, the condition of rats in the experimental group were getting worse obviously.mPAP and RV/(LV+S) were both increased (both P < 0.05). The observation by light microscope revealed that obvious myocardial hypertrophy and structure disturbances, severe luminal stenosis of medium/small pulmonary arteries, medial thickening, infiltration of inflammatory cell in tissue space, proliferation of unorganized collagen fibers in the experimental group. The observation by electronic microscope showed proliferation of endothelial cell with irregular nuclei, increased organelles and vacuoles in the experimental group. The differences in SMC, PAMT, WT%, WA%, AD, and MND were significant between two groups (all P < 0.05). ConclusionsThe monocrotaline can induced pulmonary hypertension and right ventricular hypertrophy. The mechanism may be related to severe stenosis or occlusion of the vessel lumen caused by plexiform proliferation of endothelial cells, proliferation of smooth muscle cells and collagen fibers, compensatory hypertrophy and hyperplasia of myocardial cells.