The oxygen consumption of the coelomic oöcytes of Phascolosoma vulgare, at all stages of their growth, has been studied with the diver technique. The exogenous respiration (in the coelomic plasma) shows two peaks. Each of them corresponds to a period of accumulation of nucleic acids, stainable with pyronine, in the cytoplasm. These two phases of intense oxidative metabolism seem to be coupled with the synthesis of special proteins and of polysaccharides. The endogenous respiration (in pure sea water) behaves in the opposite manner: it decreases when exogenous respiration increases and vice versa. The relation between these two variables is linear and represented by: r = −0.41 R + 10.71. The coefficient of corelation θ = −0.901 ± 0.025. The general value of this result remains to be investigated but an explanatory theory is proposed. The respiration does not recover completely in the oöcytes when they are replaced in the plasma after having been left to stand in sea water. The two peaks are eliminated completely and irreversibly during the experimental period (6 hours). This phenomenon has such a specific character that it seems to be enzymic in nature. In the biggest oöcytes sea water induces, besides this physiological change, the activation and appearance of metaphase. Three levels of oxidation have thus been recognized: endogenous respiration, fundamental respiration, i.e. respiration resistant to sea water, and extra-respiration, i.e. peaks of exogenous respiration disappearing after treatment with sea water. The significance of exogenous and endogenous conditions in oöcytes and eggs is discussed briefly.