INTRODUCTION Recent Legislative actions designed to control the levels of sulfur dioxide (SO2) in the environment have emphasized the need for better knowledge concerning the biological effects of this compound. Reports based on faulty observations have been widely used as bases for legal standards. Adequate measurement of cause and effect has been a major problem. Retrospective epidemiological surveys of pollution incidents have failed to demonstrate that SO2 is causally related to deleterious health effects. Animal experiments have yielded little significant information. This report is the third of a series in an investigation@ supported by the American Petroleum Institute@ designed to quantify effects in humans resulting from uninterrupted exposure to controlled atmospheres containing SO2. Prior reports (Refs. 1 and 2) described the effects of exposures of healthy male subjects to SO2. results of those experiments indicated the presence of increased small airway resistance after 120 hours of exposure to concentrations as low as 3 PPM SO2. At that dose level@ no SO2 related changes were observed in other parameters including subjective complaints@ clinical evaluations@ or most pulmonary function measurements.