The levels of Hg++ in different subcellular compartments of rat kidney cortex were determined after a single subcutaneous injection of <sup>203</sup>HgCl<sub>2</sub>, 4.7 mg/kg body weight. By 30 min after injection, cortical Hg<sup>++</sup>level was 46.3 ± 6.8 µg/g protein. A maximal cortical Hg<sup>++·</sup> value of 429 ± 49 µg/g protein was reached 4 h after injection. Hg<sup>++</sup>associated with the brush border membrane showed a progressive increase during the first 15 h after injection, but at each time interval was significantly less than cortical Hg<sup>++</sup> The 45-hour values for cortex and brush border were equivalent. No evidence for the presence in plasma or kidney tissue of free Hg<sup>++</sup> was obtained. Saline loading protected animals against the development of renal failure and resulted in significant lowering of cortical Hg<sup>++</sup> levels without an effect on blood Hg<sup>++</sup> concentrations.