Using an experimental model of pulmonary hypertension, the effects of anticonstrictive drugs on the development of pulmonary vascular remodeling and right ventricular hypertrophy were studied. Male Sprague-Dawley rats weighing 200~250 gm were used. For the experimental model of pulmonary hypertension, a group of animal was given by a subcutaneous injection of monocrotaline on a dose of 20mg, 40mg, or 60mg per kg of body weight. After 4 weeks of injection, all animals were sacrificed. Another group of animal was given by a subcutaneous injection of monocrotaline in a dose of 40 mg per kg of body weight. The animals were sacrificed, in which they were kept alive for 1, 2, 3 and 4 weeks, respectively. For the effects of anticonstrictive drugs on the development of pulmonary vascular remodeling and right ventricular hypertrophy, the animals treated with monocrotaline were given daily by an intraperitoneal injection of phenoxybenzamine in a dose of 1.3mg/kg of body weight, and were given propranolol via their drinking water at a concentration of 400mg/liter. The animals were sacrificed after 4 weeks of administration. The hearts and lungs were examined histopathologically and morphometrically. The results obtained were summarized as follows: 1. The rats treated with monocrotaline showed an interstitial pneumonitis, medial thickening of the pulmonary small arteries and hypertrophy of the right ventricular wall. 2. The medial thickening of the pulmonary arteries in rats treated with monocrotaline was due to muscular hypertrophy and hyperplasia, and the right ventricular hypertrophy was due to hypertrophy of cardiac muscles. Both medial thickening of the pulmonary arteries and hypertrophy of right ventricular wall were more marked with time and with dose. 3. The daily intraperitoneal injection of phenoxybenzamine suppressed significantly the percentage medial thickness of pulmonary small arteries and the index of right ventricular hypertrophy in rats given a single subcutaneous injection of monocrotaline, but propranolol has shown no protective effect on the development of medial thickening of pulmonary arteries and right ventricular hypertrophy in treated with monocrotaline. The results described above suggested that monocrotaline is an alkaloid selectively inducing pulmonary hypertension and that a-adrenergic receptor is responsible for the pathogenesis of monocrotaline induced pulmonary hypertension in rat.