Changes in weight and water content of the hepatopancreas and ovary of Crangon crangon are described in relation to body weight and ovarian development. Percentage of water content of the organs in relation to ovarian stage was determined for 43 females; organ wet weight data were obtained from another 137 animals. All stages of gonadal development (oogenesis, previtellogenesis, vitellogenesis, and depleted) were represented in the sample population of ovigerous, nonovigerous, and postovigerous females. The ovary and hepatopancreas progress through cycles of progressive and retrogressive changes in weight and volume which are characterized by a phase shift between the two organs. This resembles a Volterra-Lotka system, for which equations were derived. Wet weight of the ovary increases by 69% through previtellogenesis, and by 282% from pre- to secondary vitellogenesis. This represents an increase of 5.5% of body weight occupied by the ovary during development. The hepatopancreas nearly doubles in size from early oogonial development through previtellogenesis, but decreases markedly during vitellogenesis. The net result is an increase from a minimum value of 4% to a combined organ volume occupying 9.5% of total body weight when the ovary is full vitellogenic. Increase in ovarian mass during development is not due to water increase. Water content decreases from > 70% in immature ovaries to 46% in vitellogenic organs. The hepatopancreas also loses water, but to a lesser extent: from 73% in oogenic individuals to 67% in vitellogenic animals.