Electron-probe microanalysis was employed to follow ion concentration changes during regulatory volume decrease. The measurements were performed on isolated tubule bundles, which were dissected from medullary rays of New Zealand White rabbit kidneys. Cell swelling and subsequent regulatory volume decrease were induced by incubating the bundles in 190 mosM medium; control bundles were incubated in 290 mosM medium. Under both conditions, the detectable ions accounted for approximately 80% of the intracellular osmolarity. All cells lost significant amounts of Na, K, and Cl during cell volume regulation. While in the proximal straight tubule more than one-half of the ions lost were Na and Cl, in principal and intercalated cells of the cortical collecting duct and in the thick ascending limb the losses of Na and Cl played only a minor role. The efflux of Na and K greatly exceeded the Cl efflux, suggesting the loss of an additional undetectable anion (bicarbonate). Separate measurements in the nucleus, cytoplasm, and several cellular organelles revealed some inhomogeneity of the subcellular ion distribution.