Oxygen gas exchange was monitored in the unicellular green alga Chlorella vulgaris 211 - 11 h by means of a mass spectrometer equipped with a special membrane gas-inlet-system and a photosynthetic reaction vessel. CO 2 -dependent 18 O 2 -uptake as well as 16 O 2 -evolution were analyzed in both High- and Low-CO 2 cells. In High-CO 2 cells, the 18 O 2 -ruptake in the light (U L ) decreased by 65% upon addition of 3 mᴍ NaHCO 3 , while 16 O 2 -evolution (E) was increased approx. 1.8 times by the same treatment. 18 O 2 -uptake in the dark (U D ) was not affected by the addition of external inorganic carbon (Ci). The addition of 3.3 mᴍ NaHCO 3 also affected U L and E in Low CO 2 -cells, however, to a minor extent. U L under CO 2 -saturating conditions was light intensity-independent up to 2 klux and 1.2 klux in High- and Low-CO 2 cells, respectively. Above these light intensities U L increased approx. 4-fold in High- and approx. 6-fold in Low-CO 2 cells. Under CO 2 -limiting conditions, however, U L increased in High-CO 2 cells even under very low light intensities, showing that photorespiratory oxygen uptake occurred even in the near vicinity of the light compensation point. Under C02-saturating and strong light conditions U L represented almost half of E in Low-CO 2 cells and about 30 % of E in High-CO 2 cells. In Low-CO 2 cells addition of ethoxyzolamide (EZA), an inhibitor of carbonic anhydrase, enhanced U L and suppressed E and NET under CO 2 -limiting conditions, whereas the compound had only a minor effect on High-CO 2 cells. DCMU (3 μᴍ) strongly inhibited E and U L under CO 2 -saturating conditions, with the remaining U L being smaller than U D . KCN (1 mᴍ) and SHAM (1.5 mᴍ) added to DCMU-treated Low-CO 2 cells suppressed U L by approx. 50 % . The resulting value corresponded to half of U D . KCN also inhibited E under CO 2 -saturating conditions, with U L being strongly enhanced showing a maximal uptake at 0.4 mᴍ KCN . Under these conditions NET was nearly zero. The effect seems to be due to an inhibition of RubisCO and an enhancement of Mehler reactions. At 0.7 mᴍ KCN , DCMU entirely inhibited U L , but oxygen uptake appeared increased after turning the light off. This uptake corresponded to approx. 60 % of U D . Whereas KCN and SHAM inhibited approx. 70 % of U D , only 16% of U L was suppressed. These results suggest that the contribution of mitochondrial respiration to U L was negligeable, since U L seemed to be suppressed in the light under CO 2 -saturated conditions. Iodoacetamide, which is an inhibitor of the Calvin cycle and thereby diverts carbon into the respiratory pathway, inhibited E and NET under CO 2 -saturating conditions, but did not affect U L . This result also shows that U L is not due to mitochondrial respiration. A hydroxylamine derivative [20, 21] which changes the ratio of the RuBP carboxylation to oxygenation activity in tobacco leaves did not affect this ratio in Chlorella.