To investigate the involvement of the immune mechanism in the development of pulmonary hypertension in rats given monocrotaline, pulmonary hypertension was induced in rats and the effect of daily injection of prednisolone was studied. Fifty six Sprague-Dawley rats (4 week-old males, 50-60 g) were divided into four groups as follows: (1) injection of monocrotaline alone (A: 17 rats), (2) injection of monocrotaline + long-term injection of prednisolone (B: 22 rats), (3) long-term injection of prednisolone (C: 7 rats) and, (4) long-term injection of physiological saline solution (D: 10 rats). The survival time was prolonged, and slightly better results in the pressure of the right ventricle, hypertrophy of the right ventricle and lesions of lung tissue were obtained in B than in A. However, the rise in the RV pressure the hypertrophy of the right ventricle in B were more pronounced than in C and D, and findings in the lung tissue closely resembled those of A. Judging from the above, it is certain that long-term injections of prednisolone improve the lung tissue lesions and prolong the life of monocrotaline induced pulmonary hypertensive rats. However, it is unlikely that the immune reaction plays a leading role in the development of pulmonary hypertension in the rats.