Calcium (and other ion) transport in the thick ascending limb of Henle's loop is being studied in isolated perfused segments of rabbit renal tubules in vitro. Isotopic flux studies and electron probe microanalysis of tubular fluid reveal that significant amounts of calcium are transported and that a large component of this transport may be passive secondary to transepithelial voltage. BIBLIOGRAPHIC REFERENCES: Burg, M. and Stoner, L.: Renal tubular chloride transport. Am. Rev. of Physiology. 38:37-45, 1976. Burg, M.: Renal tubular chloride transport. Circulation. 53:587-588, 1976.