A prototype apparatus for making determinations of oxygen exchanges under controlled conditions of water flow is briefly described and some problems of technique are discussed in detail. The results include determinations of the photosynthesis and respiration of Ranunculus pseudofluitans and Potamogeton pectinatus in natural waters at velocities between 0.02 and 0.5 cm/sec, and some examination of effects of changes in irradiance and oxygen concentration. Flow was laminar at all velocities. At low velocities photosynthesis increased rapidly with velocity, but the rate of increase became less at higher velocities. The size of the effects varied with the metabolic capacity of the plant. For healthy shoots of R. pseudofluitans the maximum rate of photosynthesis was six times the probable static rate. These velocities are smaller than those in open water in streams, or even in the littoral of lakes, but may be comparable with the velocities within weed-beds.