It has been shown that morphine administration decreases brain testosterone levels probably through the activation of 5alpha-reductase (5α-R) enzyme in the brain. Recently, we indicated that finasteride, a 5α - reductase inhibitor, potentiates antinociceptive effects of morphine and prevents the development of morphine tolerance. In this work, we studied the effects of acute (10 mg/kg, i.p.) and chronic (20 mg/kg, i.p. once daily for 4 days) morphine on the 5α-R type 1 (5α-R1) mRNA levels in the adult male rat brain, using an RT-PCR method, in addition to the determination of brain testosterone levels 30 minutes following morphine. The results of our study revealed an increase in the 5α R1 mRNA levels following both acute and chronic morphine administration. It was also founded that testosterone concentration in the brain is reduced 30 minutes following morphine administration. It was concluded that the decreasing effect of morphine exposure on the brain testosterone is through an increase in the brain 5α-R1 mRNA levels. Moreover, tolerance to this effect is not developed in chronic morphine administration. Therefore, morphine may play a regulatory role in metabolism of neurosteroids, especially testosterone, in the brain which may occur at transcription level and/or translation level.This abstract will also be presented as an Oral Paper Presentation. Refer to the daily Schedule-At-A-Glance for presentation time and location. It has been shown that morphine administration decreases brain testosterone levels probably through the activation of 5alpha-reductase (5α-R) enzyme in the brain. Recently, we indicated that finasteride, a 5α - reductase inhibitor, potentiates antinociceptive effects of morphine and prevents the development of morphine tolerance. In this work, we studied the effects of acute (10 mg/kg, i.p.) and chronic (20 mg/kg, i.p. once daily for 4 days) morphine on the 5α-R type 1 (5α-R1) mRNA levels in the adult male rat brain, using an RT-PCR method, in addition to the determination of brain testosterone levels 30 minutes following morphine. The results of our study revealed an increase in the 5α R1 mRNA levels following both acute and chronic morphine administration. It was also founded that testosterone concentration in the brain is reduced 30 minutes following morphine administration. It was concluded that the decreasing effect of morphine exposure on the brain testosterone is through an increase in the brain 5α-R1 mRNA levels. Moreover, tolerance to this effect is not developed in chronic morphine administration. Therefore, morphine may play a regulatory role in metabolism of neurosteroids, especially testosterone, in the brain which may occur at transcription level and/or translation level. This abstract will also be presented as an Oral Paper Presentation. Refer to the daily Schedule-At-A-Glance for presentation time and location.