The influences of pregnancy on the pulmonary circulation in mitral stenosis (MS), were studied. 1) Twenty-two cardiac patients underwent serial right heart catheterization with the use of flow-directed catheter without X-ray fluoroscopy during the course of pregnancy. 2)In 23 cases of MS, in whom cardiac catheterization had been performed before pregnancy, clinical courses during the pregnancy were studied. In the 26-32 weeks' gestation: in MS (Group I), an elevation of peak systolic pulmonary arterial pressure (s-PAP) (mean + 14.9 mmHg, P less than 0.001) was seen; in contrast, in atrial septal defect (Group II), s-PAP remained constant. Right atrial pressure rose in the 26-32 weeks, gestation in both the groups. Infusion of 200 ml of dextran in 12-16 weeks, gestation induced an elevation of s-PAP in Group I, especially in those who fell into heart failure in the succeeding courses of pregnancy; but not in Group II. In MS, in 12 cases, whose s-PAP before the pregnancy was below 35 mmHg, no heart failure occurred during the pregnancy; in 6 of 8 cases with s-PAP between 35 and 50 mmHg, mild heart failure occurred; and in 2 cases with s-PAP over 50 mmHg, severe heart failure occurred during the pregnancy. In conclusion, in MS, PAP rises during pregnancy, and S-PAP before pregnancy and the reaction to dextran infusion can give suggestions for allowing pregnancy.