The intent of this task was to examine the toxic effect of different dosages of Duprost (dutasteride) on the spleen of mice. Total 24 albino mice were purchased and fractioned into 3 parts: control (3 mice), acute group (12 mice) and chronic group (9 mice). The acute group was subdivided to 4 groups, each of which contained 3 mice. Every group was given a lonely oral dose of the following doses: subgroup 1 dosed with 0.25 ml (0.5 mg/kg) of Duprost (dutasteride), subgroup 2 dosed with 0.15 ml (0.12 mg/kg) of Duprost (dutasteride), subgroup 3 dosed with 0.1 ml (0.08 mg/kg) of Duprost (dutasteride) and subgroup 4 dosed with 0.05 ml (0.04 mg/kg) of Duprost (dutasteride) for 24 hours. Whereas the chronic group was subdivided into 3 subgroups and each set was given a daily dose of 0.15 ml, 0.1 ml, and 0.05 ml of Duprost (dutasteride) respectively for 42 days. After the mentioned periods, the mice of all groups were sacrificed and the spleen of each animal was removed, processed, sectioned and stained for histological analysis. In the acute group, all mice that were dosed with 0.25 ml dose passed after 15 minutes of dosing. The histological analysis of spleen in residual mice of acute subgroups showed marked congestion and edema. In comparison with chronic subgroups in which depletion of white pulp had been noticed in addition to the absence of follicles and decreased the size of the periarterial lymphatic sheaths (PALS), irregular bands of fibrosis and early collagen deposition causing distortion of the splenic contour is observed. Moreover, there was an expansion of red pulp with increased cellularity and size of the red pulp. And the separation of splenic cords by irregularly shaped wide sinusoids.