By Celite chromatography and radioimmunoassay, the levels of free and conjugated forms of androstenedione, testosterone, 11-oxotestosterone, 11 beta-hydroxytestosterone, 17 beta-estradiol, and estrone have been determined in the plasma of the protandrous sex-reversing Rhabdosargus sarba at different sexual phases throughout the seasonal reproductive cycle. Free androstenedione showed a significant increase during the spawning period in the functional male phase, while a prespawning increase in both the free and the conjugated androstenedione was found in the female phase. The level of testosterone remained fairly constant throughout the reproductive cycle in all sexual phases except the intersexual phase, which was found to have a slight increase in the free testosterone level during the spawning period. Females showed a prespawning increase in free 17 beta-estradiol and conjugated estrone; the former dropped while the latter remained unchanged in the succeeding spawning period. A prespawning rise of conjugated 17 beta-estradiol was also found in the male specimens. The level of free estrone was undetectable in the plasma of Rhabdosargus. No variation in the levels of 11-oxotestosterone and 11 beta-hydroxytestosterone was found in the different sexual phases throughout the reproductive cycle. The present study showed that in general the steroid patterns in the male and intersexual phases were basically similar, except perhaps that the levels of conjugated 17 beta-estradiol, conjugated androstenedione, and free testosterone were slightly higher in the intersex than in the male during the spawning period. The changes in the plasma levels of sex steroids were correlated with the structural transformation in the gonad and the seasonal cycle of the fish. The results were analyzed with special references to the phenomenon of natural sex reversal and to the endocrine patterns known in other teleost fishes.