Pulmonary hypertension has been induced in rats by feeding them the seeds of Crotalaria spectabilis for 34 days, when some were killed and others left to recover for 14, 31 or 54 days. By 34 days and after pulmonary arterial injection, right ventricular hypertrophy was demonstrated with an increase in pulmonary arterial medial muscle, a reduced number of small arteries and “ghost” arteries. With recovery, right ventricular hypertrophy had regressed by 31 days and by 54 days the arterial muscle was decreased. There was no increase in number of small arteries although “ghost” arteries had disappeared by 31 days. Similar features have recently been described in human primary pulmonary hypertension.