[en] The process of photosynthesis in green plants is the major determinant of crop yield. Although the effects of air pollutants, such as sulfur dioxide, on photosynthesis has been studied, many unsolved questions remain. This is especially true with regard to reduction of photosynthetic rate under conditions of chronic exposure causing little or no visible injury. It was the purpose of these studies to develop techniques suitable for measuring photosynthetic rates of field-grown plants without dramatically altering the microenvironment of the plants. Gross photosynthetic rates of soybeans (Glycine max. cv. Wayne) in the field were measured by exposing a small section of representative leaves for 30 seconds to ¹⁴CO₂ in a normal atmospheric mixture by a technique similar to that of Incoll and Wright. A 1-cm² section of the area exposed to ¹⁴CO₂ is punched from the leaf and processed for liquid scintillation counting. Since the treatment period is of such short duration, there is little photorespiratory loss of ¹⁴CO₂, and thus, the amount of ¹⁴C fixed in the leaf can be related to the gross photosynthetic rate. Other parameters measured during the course of these experiments were stomatal resistance, light intensity, leaf water potential, and air temperature