In a study on metabolic consumption of photosynthetic electrons and dissipation of excess light energy under water stress, O2 and CO2 gas exchange was measured by mass spectrometry in tomato plants using 18O2 and 13CO2. Under water stress, gross O2 evolution (E(O)), gross O2 uptake (U(O)), net CO2 uptake (PN), gross CO2 uptake (TPS), and gross CO2 evolution (Ec) declined. The ratio P(N)/E(O) fell during stress, while the ratios U(O)/E(O) and E(C)/TPS rose. Mitochondrial respiration in the light, which can be measured directly by 12CO2 evolution during 13CO2 uptake at 3000 microl l(-1) 13CO2, is small in relation to gross CO2 evolution and CO2 release from the glycolate pathway. It is concluded that PSII, the Calvin cycle and mitochondrial respiration are down-regulated under water stress. The percentages of photosynthetic electrons dissipated by CO2 assimilation, photorespiration and the Mehler reaction were calculated: in control leaves more than 50% of the electrons were consumed in CO2 assimilation, 23% in photorespiration and 13% in the Mehler reaction. Under severe stress the percentages of electrons dissipated by CO2 assimilation and the Mehler reaction declined while the percentage of electrons used in photorespiration doubled. The consumption of electrons in photorespiration may reduce the likelihood of damage during water deficit.