We have previously demonstrated that magnesium therapy attenuates monocrotaline-induced pulmonary hypertension and right ventricular hypertrophy in rats. In this report we evaluate the effect of oral magnesium therapy on the pulmonary vasculature of monocrotaline-treated rats. Lung sections were prepared for light microscopic examination, and the medial wall thickness of pulmonary arteries of less than 100 microns in diameter was measured. The mean medial wall thickness of control rats was 6.98 +/- 1.16% of the external diameter of the pulmonary artery, the monocrotaline group had a significant increase in medial wall thickness (14.61 +/- 1.52%, p less than 0.005 vs control); in contrast 75% of MCT + magnesium group revealed a significant reduction in medial wall thickness (8.13 +/- 1.26%, p less than 0.02, vs monocrotaline group). Magnesium therapy alone had no effects on pulmonary vasculature. We conclude that magnesium has significant beneficial effects on monocrotaline-induced pulmonary hypertension and the accompanying vascular lesions.