There is no adequate biological information for nickel nanopowder (NiNP) in vivo. Furthermore, this study aimed to estimate the lethality percentiles doses, bio-accumulation of NiNP and evaluation of its toxicity in male albino rats. The median lethal dose (LD₅₀) of intra-peritoneal injected ultrasonicated NiNP were 3.38, 1.72, 0.87 and 0.39 g/kg body weight at 24, 48, 72 and 96 hours, respectively. Rats were daily injected with a sub-lethal dose of 0.02 g of NiNP /kg body weight/day (1/20 of LD₅₀ at 96 hours) for 28 days. The nickel accumulated in the brain, liver, kidney and intestine showed a direct relationship with the experimental periods, and their levels were significantly higher than the corresponding controls. The brain, hepatic and renal malonedialdehyde (MDA) and oxidized glutathione (GSSG) levels exhibited a positive relationship with NiNP accumulated in these tissues, whereas the glutathione (GSH) and GSH/GSSG ratio showed a negative relationship. In comparison with the corresponding controls, the levels of GSH and GSH/GSSG ratio decreased significantly and accompanied with marked elevation of MDA and GSSG. After 28 days post-injection, the levels of total proteins (TP), albumin (Alb) and high-density lipoproteins (HDL) in serum were strikingly lesser than the corresponding controls, and were associated with a prominent elevation of urea, creatinine, triglycerides (TG), total cholesterol (TC), low-density lipoproteins (LDL) and activities of transaminases (AST and ALT). The levels of TP, Alb and HDL exhibited an inversely relationship with the experimental periods, whereas levels of urea, creatinine, TC, LDL and activities of AST and ALT showed a positive correlation. In conclusion, bioaccumulation of NiNP was time-, dose- and tissue- dependent, and are moderately able to induce oxidative damage to the mammalian tissues.