The aim of this study is to investigate the possible protective effect of N-Acetylcysteine (NAC) against the likely methotrexate (MTX) toxicity on the kidney using ultrastructural together with biochemical data. Moreover, the immunohistochemical detection of Ki67 nuclear antigen is to be evaluated. Fifteen male Wistar albino rats, weighing 240-290 g, were divided into three equal groups: Rats receiving MTX alone, rats receiving MTX plus NAC treatment, and rats comprising the control group. MTX (18 mg/kg/day, body weight) in dissolved physiologic saline was administered intraperitoneally to rats during 3 days. For the MTX plus NAC group, N-Acetylcysteine (300 mg/kg/day, body weight) was administered together with MTX. At the end of the third day, all the rats were killed with cervical dislocation to obtain blood and tissue samples. Application of MTX principally induced prominent large vacuolization in the proximal convoluted tubule cells, and focal thickening in the glomerular basal lamina of some glomeruli. A decrease in tissue SOD (superoxide dismutase) and GSH-Px (glutathione peroxidase), and an increase in serum urea nitrogen and creatinine and in tissue MDA (malondialdehyde) levels were also seen in the MTX group. These changes were significantly reversed in the MTX-plus-NAC-treated group. Most of the vacuoles in the proximal convoluted tubule cells disappeared. Furthermore, an increase in antioxidant enzyme activities, a decrease in serum urea nitrogen and creatinine, and tissue MDA levels were all significant. Additionally, an increase in the number of Ki67 positive-stained cells in proximal tubules was also noted. In conclusion, NAC may be a promising substance against MTX-induced renal damage. It might be useful to use NAC supplementally to minimize MTX-induced nephrotoxicity.