Emphysema was produced in 8 of 40 rats exposed to either a papain aerosol alone or accompanied by oral ad- ministration of penicillamine. Penicillamine medication did not enhance the ability of papain to produce emphysema. The average mean right ventricular systolic blood pressure was elevated in the 8 emphysematous rats and 2 of them developed hypertensive pulmonary vascular disease, but no right ventric- ular hypertrophy. The pulmonary vascular lesions consisted of abnormal muscularisation of pulmonary arterioles and medial thickening of the smaller muscular pulmonary arteries. Morphometry was used to measure the mean linear intercept (MLI), surface density of lung tissue (St), and the surface to volume ratio of the alveolar spaces (Sa/Va). The mean right ventricular systolic pressure was related to the PaO2, MLI, St, Sa/Va, and the percentage of abnormal air space in the lung. The study supports the hypothesis that the pulmonary hypertension which occurs in emphysema is due to alveolar hypoxia and not to reduction of the pulmonary capillary bed