A randomized, double blind and placebo-controlled pilot study was done on the use of converting enzyme inhibitor captopril in 98 patients in the early stage of acute myocardial infarction (AMI) with the purpose of testing the feasibility and justifiability of a large clinical trial on such therapy. The results of the study were as follows. (1) The randomization in this study was quite satisfactory. The interruption of the treatment protocol for more than 24 hours and the patients drop out rate were only 5.1% each, and no side effects of captopril were observed. These indicated that such study could be carried out on large scale. (2) Captopril slightly lowered blood pressure (average 10.2/4.5mm Hg after 2 hours of oral 6.25mg) with minimal change of heart rate, but it did not increase the incidence of sustained hypotension. (3) Captopril decreased the angiotensin II level significantly after 2 hours of ingesting 12.5mg. (4) Early captopril treatment tended to reduce the incidence of cardiac free wall rupture, the severity of congestive heart failure and cardiogenic shock, therefore tended to reduce the early mortality of AMI (3/43 captopril group VS 8/55 placebo group).