BACKGROUND: Although the long term adverse effects of cigarette smoking on health are well known, the acute possible detrimental effects of smoking on pulmonary or cardiovascular function, especially when these systems are stressed by the metabolic demands of exercise, have not been well studied. The purpose of this study is to .determine the acute action of cigarette smoking on cardiopulmonary function under stress.' METHOD: Twenty-one healthy smoking subjects were studied. Before exrecise testing, history taking, physical examination and baseline studies, including CBC, chest PA, PFT and EKG, were done. The subjects performed an incremental bicycle exercise test to exhaustion on two occasions, one without smoking and the other after smoking 5 cigarettes/h for 2 hours. All indices of P.F.T and bicycle ergometry were compared between before and after smoking. RESULTS: 1. VO2max and O2 pulse showed significant decrease in smoking day. 2. Although there were no significant differences, anaerobic threshold showed a tendency of decrease and HRmax showed that of increase in smoking day. 3. P.F.T. and respiratory indices showed no significant change in smoking day. CONCLUSION: Cigarette smoking has immediate adverse effect, especially on the cardiovascular system rather than the respiratory system. These results would be due to the effect of elevated HbCO and/or impaired blood flow iii response to the exercise stimulus.