The K+ and Cl- concentrations of the tube foot fluid in intact starfish (A. forbesi) are higher than that in the external seawater or perivisceral (coelomic) fluid. The tube foot fluid is distinctly hyperosmotic with respect to the seawater environment. Increase in K+ in the tube foot lumen fluid is brought about by the active secretion of K+ by the epithelium, with Cl- following passively. This secretion of K+ is electroneutral. No exchange of material in the external environment with the more distal portions of the water vascular system (tube feet) across the madreporite was observed. Fluid volume in the tube feet is probably maintained by active K+ secretion by the tube feet epithelium creating an osmotic driving force for water into lumen of the tube feet.