A number of cells, e.g. muscle and nerve of the blue crab Callinectes sapidus, will swell when exposed to a hypo-osmotic stress and re-adjust their volumes back toward the original level while still maintaining internal osmotic pressure equal to the now diluted medium. This process involves amino acid metabolism and/or transport. The object of this study is to: a) characterize volume and cell membrane potential control, b) determine what molecules and mechanisms are involved, and C) show how this process fits in the normal physiological responses of this animal. Among the techniques used will be: whole animal a) equilibration to closed environments ranging from 100 mosm/kg to 2000 mosm/kg, b) body fluid analysis for osmotic pressure, amino acid composition and sodium activity, c) isotope flux studies from muscle bundles, d) electrical properties of cell membranes, e) intracellular ionic activities, and f) cellular morphology. The hypothesis to be investigated is that amino acid transport is a metabolic process thermodynamically coupled to ionic gradients.