Gaseous metabolism was studied throughout an entire reproductive cycle. Variations in the O2 requirement of a full-grown mussel under the factors, both physical and physiological, normally incident upon the mussel-beds in the course of a year, were determined, and quantitative expressions obtained for the effect of temperature and O2 pressure on the rate of O2 uptake, which is also profoundly affected by the state of reproductive maturity. Influence of the annual temperature wave is approximately 3 times that attributable to the sexual cycle. Seasonal changes in absolute O2 requirement, and the respiratory quotient, are intimately associated with concurrent changes in the chemical composition of the tissues.