The effects of habitual cigarette smoking on cardiorespiratory responses to sub-maximal and maximal work were evaluated in nine adult nonsmokers and nine smokers with a mean age of 33 yr. A maximal treadmill test was followed by three tests at 45, 60 and 75% of each subject's VO₂max. Compared to nonsmokers, the habitual smokers had a non-significantly lower VO₂max in L/min and per lean body mass (9 and 6%, respectively), but had higher %fat (p<0.01), resulting in a significantly lower VO₂max per kg body wt (13%, p<0.03). Maximal exercise ventilation (V_E) was 16% lower in smokers. During sub-maximal work at equivalent exercise stress levels in the two groups, the V_E/VO₂ ratio was higher in smokers by an average of 11% because VO₂ was lower and the respiratory exchange ratio values were significantly elevated in smokers at 75% of VO₂max. Blood lactate concentrations in smokers were higher as workloads increased and O₂ pulse (VO₂/HR) was significantly lower throughout, indicating reduced O₂ extraction, probably due to carbon monoxide. The resting HR was significantly higher in smokers and the HR recovery following all three submaximal exercises was significantly slower in smokers. These results show that detrimental cardiorespiratory effects of chronic cigarette smoking in apparently healthy individuals are evident at moderate exercise levels as reduced gas exchange efficiency in lungs and muscles.