To evaluate the degree of pulmonary artery hypertension (PAH) and its significance, pulmonary perfusion tomographic imaging (PPTI) and radioimmunoassay of plasma atrial nateiuretic. peptide (ANP) levels were examined in different stages of rabbit PAH models, controlled with cardiac catheterization. When the PAH was mild. ANP levels were not significantly changed (t=1, p >0.05). The mean pulmonary artery pressures (mPAP) measured by catheterization had no significant change, but there was a significant increase of the ratio of dorsal/abdominal counts using pulmonary perfusion tomographic imaging (PPTI) (t=2.5, p <0.05). The ANP levels rose when PAH was moderate or severe, and the difference was significant compared with the control group (t=4 and 6.5, p <0.05). The other two methods also showed significant changes (p <0.01). There was positive correlation between the results of ANP/PPTI, ANP/catheterization, and PPTI/catheterizations (p <0.01). These results suggest that ANP levels can assess the degree of the PAH as a simple method, but it is not as sensitive as that of the PPTI in mild PAH.