OBJECTIVE: To observe the effects of the L-Arginine in the nephrotoxicity (induced by Lithium carbonate) in albino rats. DESIGN: A prospective experimental study. PLACE: Department of Anatomy, Basic Medical Sciences Institute, Jinnah Post Graduate Medical Centre Karachi DURATION: July 2007 to November 2007 METHODOLOGY: Forty five adult albino rats of either sex were taken and divided into three groups as A, B and C, which were further divided into three subgroups according to period of the treatment they received i.e. two, four and six weeks respectively. Group A animals served as control, Group B received lithium carbonate (Neurolith, Adamjee Pharma) at dose of 20 mg/kg/day with feed and Group C animals received lithium carbonate 20 mg/kg/day with feed plus L-Arginine (Arginine, General nutrition corporation, Pittsburg USA) 300 mg/kg/day with feed. After end of their respective period of treatment animals were sacrificed and their kidneys were removed, fixed in alcoholic formalin, and after processing were embedded in paraffin and 5μ thick sections were cut and stained with PAS-Haematoxylin. RESULTS: PAS-Haematoxylin stained sections revealed significant and highly significant (i.e. P < 0.05, P < 0.001) decrease in proximal tubular count and proximal tubular nuclear count in sub group B2 and B3 animals respectively. A significant and moderately significant (P < 0.05, P < 0.01) decrease was also noted in nuclear diameter of proximal tubular cells in subgroup B2 and B3 respectively. Results of Group 'C' were found insignificant when compared with control group. CONCLUSION: After morphometeric study it appeared that L-Arginine minimizes the toxic effects of Lithium carbonate on proximal tubules of kidney.