Radioactive nickel chloride (63NiCl2) was injected i.v. into rabbits in dosage of 0.24 mg Ni/kg body wt. 63Ni was rapidly cleared from the serum (T1/2 ≃ 8.2 h) during the period from 1 h to 2 days after the injection, but slowly (T1/2 ≃ 95 h) during the period from 4 to 7 days after the injection. During 24 h after injection of 63NiCl2, an average of 90% of serum 63Ni was bound to albumin and 10% was ultrafiltrable. Chromatography on Sephadex G-25 demonstrated the presence of five distinct 63Ni complexes (Fractions I to V) in serum ultrafiltrates. During 24 h after injection of 63NiCl2, an average of 78% of the administered dose of 63Ni was excreted in the urine. Chromatography of the urine on Sephadex G-25 separated three 63Ni complexes, which appeared to have identical chromatographic mobilities as serum Fractions II, III, and V. The chemical identities of the ultrafiltrable 63Ni complexes in serum and urine have not been established, although one (Fraction V) resembles Ni histidine in its chromatographic mobility on Sephadex G-25. This study demonstrates that ultrafiltrable nickel in serum and urine does not exist primarily as free Ni(II), but instead as Ni complexes, and suggests that ultrafiltrable Ni receptors play an important physiological role in nickel homeostasis by serving as diffusible vehicles for the extracellular transport and renal excretion of nickel.