Copper nanoparticles are widely utilized in a variety of applications, including metal catalysts, semiconductors, heat transfer fluids in machine tools, and even in antibacterial medications. Forty mature healthy Westar rats were utilized in the current investigation and grouped randomly into four groups (n = 10 rats/group). Group I (G1) was kept as a control group, but G2, G3, and G4 were intraperitoneally injected with CuO NPs with a dose (5 mg, 10 mg, 25 mg/kg body weight/day) respectively for 9 days. Rats were sacrificed; then, the livers and kidneys were dissected and subjected to histopathological and immunohistochemical examination. Our findings of G2 and G3 revealed mild to moderate degenerative changes within the hepatic parenchyma, moderate blood vessel congestions, glycogen depletion, hemosiderosis, and microvesicular steatosis (fatty changes within the hepatocytes). In addition, at the level of kidney, our examination clarified moderate degenerations of the renal corpuscles and renal tubules with moderate swelling and congestions of the glomerulus with moderate vacuolations in the renal tubules lining epithelium. On the other hand, increasing the dose of CuO NPs, the toxicity became more obvious, where the liver of G4 revealed severe necrosis of hepatocytes with completely disorganizations of the hepatic rays, loss of the hepatic architectures, severe steatosis, severe hemosiderosis, sinusoidal dilatations with congestions, as well as severe fibrous tissue proliferation with anti-inflammatory cell infiltrations specially around portal triad with hyperplasia of bile duct. Meanwhile in kidney, G4 clarified severe necrosis and atrophy of the renal corpuscles with severe damage of Bowman's capsule leading to completely disorganization and loss of normal renal cortex architectures, severe congestion of the glomerulus, severe necrosis of the renal tubules with damage and sloughing for its lining epithelium, and severe hemorrhage between renal tubules. In addition, severe and diffuse caspase 3 immunoreactivity were observed within the hepatic and renal tissues of G4. The present investigation was concluded that the CuO NPs have a potential toxicological effect on the hepatic and renal tissues that may affect their functions.-->