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Background: Nitric oxide (NO) as a signaling molecule plays a predominant role in different physiological and pathological processes in a variety of organs such as the gastrointestinal (GI) tract. It is produced from L-arginine by nitric oxide synthase (NOS), which in turn is inhibited by L-NG-nitroarginine methyl ester (L-NAME). Objectives: This study aimed to stereologically analyze colon under exposure of L-arginine and L-NAME in female Wistar rats. Materials and Methods: In this experimental study, 40 female rats (200 - 250 g, 8 weeks age) were divided into 5 groups (N = 8). Normal saline (2 mL/kg), L-arginine (200 mg/kg), L-NAME (20 mg/kg) and L-arginine + L-NAME (with the same doses) were administered i.p. for 3 days. After 2 weeks, the colon was removed, stained with hematoxylin and eosin (H and E) and evaluated under light microscopy. Colonic mucosal thickness, gland height and diameter, and total thickness of the wall were measured using optical software and analyzed by one-way ANOVA followed by Tukey's post hoc test using SPSS-16. P < 0.05 was considered statistically significant. Results: The data of the present work revealed relatively narrow ranges of decrease in mean variables compared to the control group, which was not significant. Conclusions: Despite the remarkable role of NOin GI tract, the results demonstrated no significant effect of L-arginine and L-NAME on colon parameters. [ABSTRACT FROM AUTHOR]