Photosynthesis by Podostemum ceratophyllum in closed production chambers became inhibited by oxygen accumulation and carbon depletion during field investigations. Carbon-14 uptake by this plant during 180-min experiments was initially rapid, then decreased abruptly. The photosynthetic response corresponded to increased oxygen concentration of the chamber water and increased excretion of labelled organic carbon from the plants. Photosynthesis was probably further inhibited by inorganic carbon depletion since this plant is unable to use HCO//3** minus as a carbon source. Alkalinity and pH in the chambers decreased and increased, respectively, reflecting this depletion of available carbon dioxide. These data suggest that use of sealed chambers for aquatic macrophyte production studies may seriously underestimate actual production.