The results of this study indicate that both cell proliferation and increased synthesis of extracellular matrix protein contribute to hypertrophy of the rat pulmonary trunk during the early development of hypoxia-induced pulmonary hypertension. As determined by autoradiography after <sup>3</sup>H-thymidine injection, pulmonary hypertension results in increased labelling in all cell compartments of the pulmonary trunk wall, the most dramatic response occurring in the adventitia following 3 days’ hypoxic exposure. Autoradiography also demonstrated differences in the degree of incorporation of <sup>3</sup>H-proline into extracellular protein between hypoxic (3 and 21 days) and control rats. The major focus of <sup>3</sup>H-proline incorporation shifted from the adventitia at 3 days to the tunica media at 21 days, although incorporation was significantly higher at 3 compared to 21 days in all wall compartments. The patterns of hyperplasia and matrix protein synthesis in the extrapulmonary arteries of the rat, as reported here, are distinctly different from those seen in many large elastic arteries during development of systemic hypertension. For example, the hyperplastic response of arterial vessels follows a similar temporal sequence in pulmonary and systemic hypertension. However, the adventitia is the region of the pulmonary trunk with highest cell proliferation in pulmonary hypertension while the media is most affected by systemic hypertension. The relevance of the changing patterns of cell proliferation and protein synthesis in the wall of the pulmonary trunk of chronically hypoxic rats to the structural and biochemical properties of this vessel during the early development of pulmonary hypertension is discussed.