Measurements were made of variations in water, protein, and soluble nitrogen in the hepatopancreas ofCarcinus maenasduring the intermolt cycle and in the ovary during ovogenesis. Analyses of hepatopancreas show three reasons for the variations in protein and soluble nitrogen contents: entry of water into the organism during and after exuviation and a corresponding dilution of the constituents, alternation of fasting and feeding during the intermolt cycle, and heavy demands of ovogenesis resulting in a decrease in nitrogen in the hepatopancreas. Variations in the metabolism of water and nitrogen are shown in the ovary during ovogenesis.