The plasma levels of both the free and conjugated forms of six sex steroids (androstenedione, testosterone, 11-oxotestosterone, 11 beta-hydroxytestosterone, estrone, and 17 beta-estradiol) were determined by radioimmunoassay combined with Celite chromatography in different sexual phases of the protogynous Monopterus albus throughout the reproductive cycle. The amounts of 11-oxotestosterone and 11 beta-hydroxytestosterone were found to be very low and variable in all the specimens investigated. No conjugated 17 beta-estradiol or free estrone was detected. Female individuals showed a prespawning rise of androstenedione which subsequently dropped to a low level in the spawning period. The estrogen levels in the female phase were found to be higher than those in all other sexual phases during the spawning period, but the testosterone level remained constant throughout the reproductive cycle in the female phase. The level of androstenedione was highest in the early intersexual and mid-intersexual phases during the postspawning/inactive period. Compared with the female specimens in the same reproductive period, the early intersexual individuals showed a higher level of 17 beta-estradiol, while the mid-intersexual animals showed a higher level of testosterone in the postspawned/inactive period. In the mid-intersexual phase, the levels of androstenedione, testosterone, 17 beta-estradiol, and estrone dropped progressively in relation to the seasonal reproductive cycle. The male fish had a constant level of androstenedione, estrone, and 17 beta-estradiol. However, the level of testosterone increased as the spawning period approached. The hormonal profile in the late intersexual phase was essentially similar to that in the male phase. The changes in the plasma levels of sex steroids in M. albus apparently were related to the maturation of the female and male sex tissues and to their seasonal reproductive cycle.