OBJECTIVE: To evaluate the possible effects of paraquat spraying among workers on deciduous fruit farms in the Western Cape, South Africa. Paraquat is a commonly used herbicide world wide and is a well documented cause of pulmonary fibrosis in studies of laboratory animals and in humans after exposure to a high dose (usually accidental or as parasuicide). The respiratory effects of long term, low dose exposure to paraquat have not been fully evaluated. METHODS: A cross sectional study of 126 workers. Administered questionnaires generated information on exposure, respiratory symptoms, and potential confounding variables. Spirometry and gas transfer were measured and chest radiographs performed. Oxygen desaturation on exercise testing was by oximetry during a modified stage one exercise test. RESULTS: No association was found between long term exposure to paraquat and reported symptoms, spirometry (forced vital capacity (FVC), forced expiratory volume in 1 second (FEV1), FEV1/FVC) and gas transfer (TLCO and KCO) or chest radiography. Multivariate analysis showed a significant relation between measures of long term exposure to paraquat and arterial oxygen desaturation during exercise independent of short term exposure. CONCLUSION: Previous studies have not shown a significant relation between measures of exposure to paraquat and standard tests of lung function. Arterial oxygen desaturation during exercise represents a more sensitive test. The findings indicate that working with paraquat under usual field conditions is associated with abnormal exercise physiology in a dose dependent fashion independent of recent exposure and acute poisoning events.