X-ray microanalysis of frozen-hydrated specimens of the cicada filter chamber shows that there are concentration changes in K+ and Cl- in the various compartments of the filter chamber from feeding to non-feeding states. These changes are consistent with an osmotic transfer of water between the anterior midgut and the posterior midgut and Malpighian tubules. K+ and Cl- account for a large proportion of the osmotically active solutes in these compartments, and it is suggested that the osmotic gradients are generated by secretion of K+ and Cl- into the posterior midgut and Malpighian tubule lumina. Analysis of freeze-substituted sections of filter chamber supports the analysis of frozen-hydrated specimens and indicates that Na+ may also be secreted into the posterior midgut and the Malpighian tubules. The system functions as a water-shunting complex.