From parallel observations on changes in respiration rate in vivo and mitochondrial activity in vitro it is concluded that the respiration of pea cotyledons during germination is mediated by enzymes of the citric acid cycle and of the electron transfer chain. Its course may be divided into four phases. The sharp rise in respiration rate in the first phase is due to activation by hydration of the enzymes of the two systems during swelling. The fairly constant respiration level in the subsequent phase is only to a minor extent due to a limited oxygen supply but primarily to the full hydration of the enzymes of the electron transfer chain present in the airdry cotyledons with a certain potential activity. The increase in respiration rate in the third phase is attributed to a rise in activity of the mitochondrial enzymes, whereas the decrease in respiration rate in the fourth phase seems to be due to a disorganization of the subcellular structure of the cotyledons.