The present study was undertaken to test the hypothesis that chronic, low-level paraquat exposure causes restrictive lung function with gas transfer impairment. Three hundred thirty-eight Costa Rican farm workers from banana, coffee and palm oil farms completed a questionnaire, spirometry and single-breath carbon monoxide diffusing capacity. Subjects 40 years of age or older, without other medical risk factors, completed maximal cardiopulmonary exercise tests. Most (66.6%) were paraquat handlers; 24.8% of handlers and 27.3% of non-handlers reported current cigarette smoking. In linear regression models, cumulative paraquat exposure was not an independent predictor of alveolar volume, carbon monoxide diffusing capacity, peak oxygen uptake, forced vital capacity or oxygen pulse peak. However, the ventilatory equivalent for CO2, although within normal range, was significantly higher with increased cumulative paraquat exposure. Oxygen desaturation greater than 5% from rest to peak exercise had an odds ratio of 1.7 (95% confidence interval = 0.9 – 3.0) with the cumulative paraquat exposure index in models adjusted for age, weight and smoking status. The association of paraquat exposure with ventilatory equivalent and oxygen desaturation suggests that paraquat may be associated with sub-clinical gas exchange abnormalities, but overall these findings are consistent with no clinically significant increases in interstitial thickening or restrictive lung disease among this population.