Changes in activity of some mitochondrial enzymes from cotyledons of peas have been followed during germination. During the first day the activity of the succinate and malate oxidase system increased whereas succinate and malate dehydrogenase activity hardly changed. Oxidase and dehydrogenase activity both increased during the subsequent two days. The respiratory control of mitochondrial fractions from mature, air-dry cotyledons was very low but increased rapidly during the first day of germination. Mitochondrial activity decreased during the later phases of germination. It is suggested that the increase of the respiratory capacity of the mitochondria during the first day is effected by an increase of the capacity of the electron transfer chain.