The objective of this study was to investigate the effects of 17α-methyltestosterone （17α-MT） on gonadal development, serum steroids and aromatase activities in 2-year-old female red-spotted grouper（ Epinephelus akaara ） during reproductive season. Groupers were divided into two groups, one implanted with 17α-MT （ 10 mg? kg^-1 body weight） and the other elastomer without 17α- MT into peritoneal cavity once every four weeks for 8 weeks. Blood samples were obtained from 6 fish of each group every 4 weeks for later analysis of sex steroids. After blood samples were collected, forebraln, midbrain, hindbrain, and gonads were collected and stored at - 70 ℃ for later aromatase activity measurement and gonadal histological study. Significantly lower gonadosomatic index （GSI） was observed in 17α-MT-treated group. Fish implanted with 17α-MT once showed complete degradation of oocytes and sex inversion with developing testicular tissues at the 4th week, while all fish in the control group were still female with developing ovaries. 17α-MT induced females to develop into functional males with authentic testes similar in structure to those in normal males, and no significant difference in sperm motility rate and concentration was observed between them. 17α-MT elevated the aromatase activities both in forebraln and midbrain but reduced that in hindbrain, with no difference in gonad before and after 17α-MT implantation. Serum 11-ketotestosterone （11-KT） levels increased significantly on the 4th week, while no differences of serum testosterone （T） and estradiol-17β （E2） levels were observed in control and 17α-MT-treated group. Present results suggested that the elevation of brain aromatase activities was associated closely with sex reversion, and the increasing 11-KT levels may be important for sex inversion induced by 17α-MT in red-spotted grouper.