Using a manometric method, photosynthetic oxygen evolution and (14)CO(2) fixation have been determined for leaf tissue of Triticum aestivum L., Hordeum vulgare L., Phaseolus vulgaris L., and Lemna minor L. Approximately similar values in the range 0.2 to 0.4 millimoles grams fresh weight(-1) hour(-1) were obtained for both gases. In tissue subjected to vacuum infiltration, O(2) evolution and (14)CO(2) fixation were barely measurable. It is considered that the elimination of photosynthetic gas exchange results from a decreased supply of CO(2) to the chloroplasts. Chopping wheat laminae also leads to a reduction in photosynthetic gas exchange, slices 1 millimeter or less giving only 10 to 20% of the value for whole tissue. Respiration is unaffected by either treatment. Carbonic anhydrase did not improve photosynthetic gas exchange in infiltrated tissue. The use of sliced or vacuum-infiltrated leaf tissue in photosynthetic studies is discussed.