Phosphinothricin (PPT) and aminooxyacetic acid (AOA) cause a blockage in the photorespiratory pathway and consequently the photosynthesis rate is diminished. Treatments with photorespiratory inhibitors, which last more than 1 h, induce a depletion in the ratios of Fv/Fm, Fv/Fo and Fm/Fo. A good linear relation between the photochemical efficiency of PS II and the quantum yield of the CO 2 assimilation with both inhibitors was observed. With AOA treatment the photosynthetic quantum yield was practically nill while a depletion of 12 % was observed in the ratio Fv/Fm. With PPT treatment there was a similar decrease in net CO 2 assimilation and the ratio Fv/Fm. The RUBISCO activity, and not the photochemical efficiency of PS II, would probably be the cause that explains the decrease of photosynthesis rate. A relationship between the 3-PGA content in the leaves and the photochemical efficiency of PS II was observed when the interruption of photorespiration took place. We propose that the Calvin cycle metabolites, and especially those that require ATP and/or NADPH for their metabolism, as for example 3-PGA or triose-P, would control the photosynthetic electron transport capacity when the photorespiration is blocked.