[en] The development of a scintillation system for low ¹⁴CO₂ radioactivity measurements is described and its use in determining parameters of gas exchange in plants is outlined. It permits simultaneous measurement of true photosynthesis (¹⁴CO₂ absorption) and apparent photosynthesis (¹²CO₂ absorption) and hence determination of photorespiration in light. In darkness it permits simultaneous measurement of respiration (¹²CO₂ evolution) and dark carboxylation (¹⁴CO₂ absorption). Transpiration of plants can also be measured in both light and darkness. The course of gas exchange in wheat and maize plants is shown. (Auth./C.F.)