Brain sexual differentiation begins in the late embryonic period and continues for short time after birth. Sex hormones, particularly testosterone, play an important role in this process. During the critical period of brain sexual differentiation aromatase enzyme converts testosterone into estrogen which in turn, masculinizes the brain. Thus, it is likely that by inhibition of aromatase the masculinization of male fetal brain will be blocked. Material and methods:Pregnant Wistar rats were administered letrozole (0.5mg/kg) at19th and 20th days of pregnancy. When offspring were become two months old, the spatial memory of experimental male offspring were compared by age-matched male and female controls, using Morris water maze. Then their brain were fixed for histological processing. After histological preparation, the brain sections with thickness of 8 µm were stained With H&E.By using a stereological technique, the dissector, the numerical density (NV) of AVPN and Arc nucleus were estimated. Results and conclusion:In compare with male and female controls, the mean time to reach the platform on days 1-4, as a criteria for learning, and the mean time spent in target quadrant on day 5 (probe test) in experimental males were between the control male and female. These finding may indicate that the process of masculinization the male,s offspring,s brain has not been done completely. The estimated NV (×103 /mm3) of AVPV was significantly increase in experimental males. In Arc, the estimated NV was significantly decrease in experimental males. Therefore it can be concluded that by inhibition of aromatize activity and the consequential reduction of estrogen bioavailability in the brain the normal musculinization of dimorphic nucleus in the male,s offspring,s brain may be prevented.

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