Summary. Detached spruce twigs, wheat and soybean leaves were infiltrated with various metabolic inhibitors, placed in a closed system in C02-free air and the amounts of C02 evolved in either light or darkness were determined with an infra-red CO0 analyzer. In light, metabolic inhibitors always greatly suppressed evolution of C02, the magnitude of suppression varying between 50 to 80 % of that without an in'hibitor. This depressing effect became less pronounced with increasing oxygen. In darkness, metabolic inhibitors sometimes suppressed and sometimes stimulated CO2 evolution. These observations have been taken as 'further support for a conclusion made earlier, that evolution of CO2 in light and darkness is not thie same process.In our earlier work we observed that an increase in the oxygen content of air between 1 and 100 % had a different effect on the evolution of CO2 in light or darkness by detached leaves of soybean (3) 5 species of grasses (4) and tobacco (9). There was marked stimulation of COO evolution in light, but not in darkness. Similar observations have been reported by other workers (2, 5).Such a difference could be explained on the assumption that evolution of 002 by green 'plants in light and in darkness is not the same process. In light, the usual or dark respiration could either be suspended, or at least considerably curtailed, and be replaced by some other process, which we have tentatively called photorespiration. If both, photorespiration and dark respiration are the same, one expects various external conditions to have the same effect on both of them. I,f orn the other hand these 2 processes are not the same, then the effect of the same external condition may be different.The experiments reported below describe the effects of various metabolic inhibitors on CO2 evolution in light and in darkness at different concentrations of oxygen. Materials and MethodsAttached shoots as well as detached twigs of 4-year-old spruce seedlings, Picea glauica Moench/