Chromatographic and electrophoretic fractionations of 63Ni-constituents were performed upon renal cytosol samples from rats that were killed following administration of 63NiCl2. At 0.5 to one hr after i.v. injection of 63NiCl2 (0.1 to 0.5 mumole per kg body wt., N = 6), renal cytosol contained 11 percent (SE +/- 2 percent) of the 63Ni-dose. An average of 68 percent (SE +/- 4 percent) of 63Ni in these cytosol samples was associated with low molecular weight components (mol. wt. less than 2,000). The remainder was bound to five macromolecular constituents, with molecular weights of greater than 130,000 (Peak A), approximately 70,000 (Peak B), approximately 55,000 (Peak C), approximately 30,000 (Peak D), and approximately 10,000 (Peak E), based upon column chromatography on Sephadex G-200. The 63Ni that was present in Peaks A to E averaged, respectively, 1.8 percent, 10.7 percent, 7.5 percent, 10.0 percent and 1.6 percent of total 63Ni in renal cytosol. At one to fours hrs after i.m. injection of 63NiCl2 (100 mumol per kg body wt., N = 17), renal cytosol contained 2.2 percent (SE +/- 0.1 percent) of the 63Ni-dose; ultrafiltrable 63Ni averaged 57 percent (SE +/- 3 percent) of total 63Ni in renal cytosol. Macromolecular 63Ni-constituents in these samples were separated by electrophoresis in agarose gel and visualized by autoradiography. Peak D (mol. wt. approximately 30,000) was the most prominent of the five 63Ni-constituents. This study demonstrates that renal cytosol of 63NiCl2-treated rats contains five macromolecular 63Ni-constituents that may be involved in renal uptake, transport, storage, and excretion of 63Ni(II).