Plasma and gonadal levels of several gonadal steroids (testosterone, 11-ketotestosterone, androstenedione, 11 beta-hydroxyandrostenedione, 17 beta-estradiol, and estrone) were measured by RIA in the protandrous seabass, Lates calcarifer, throughout an annual reproductive cycle. Twenty to 25 fish were killed every month for gonadal and plasma sampling. Very low plasma levels of 11-ketotestosterone in females (monthly means always less than 75 pg/ml), and of 17 beta-estradiol (means always less than 68 pg/ml) and estrone (means always less than 42 pg/ml) in males did not fluctuate significantly during the cycle. Conversely, plasma concentrations of testosterone, estrone, and 17 beta-estradiol peaked during vitellogenesis in females (highest mean: 182 +/- 121, 182 +/- 32 and 598 +/- 369 pg/ml, respectively) and testosterone and 11-ketotestosterone peaked during spermiation in males (highest mean: 189 +/- 91 and 223 +/- 94 pg/ml, respectively). When sex type are compared over the whole cycle, females displayed higher 17 beta-estradiol (172 +/- 233.5 pg/ml) and estrone (79.5 +/- 72 pg/ml) levels than males (57 +/- 7.5 and 44 +/- 62.5 pg/ml, respectively), while males had higher 11-ketotestosterone levels (153 +/- 88 pg/ml) and, to a lesser extent, higher testosterone levels (128 +/- 82 pg/ml) than females (51.5 +/- 28 and 91.5 +/- 60 pg/ml, respectively). Transitional fish always exhibit low plasma levels for these four steroids (testosterone 56.5 +/- 12.5 pg/ml, 11-ketotestosterone 59 +/- 23.5 pg/ml, 17 beta-estradiol 65.6 +/- 36 pg/ml, and estrone 61 +/- 47.5 pg/ml). Among gonadal androgens, 11 beta-hydroxyandrostenedione predominated in testes (3.95 +/- 3 ng/g), except during spermiation (0.8 +/- 0.5 ng/g), and remained low in ovaries (1.05 +/- 1.4 ng/g). No differences were detected in gonads, for testosterone and 11-ketotestosterone whatever the sex type, but their concentrations were higher in vitellogenic and atretic ovaries. Androstenedione levels were slightly higher in testes (2.21 +/- 2 ng/g) than in ovaries (1.53 +/- 1.32 ng/g). Transitional gonads always showed low concentrations for these four androgens (testosterone 0.66 +/- 1.77 ng/g, 11-ketotestosterone 0.14 +/- 0.05 ng/g, androstenedione 0.3 +/- 0.34 ng/g, and 11 beta-hydroxyandrostenedione 0.2 +/- 0.23 ng/g). Gonadal 17 beta-estradiol was nearly undetectable in testes (0.06 +/- 0.07 ng/g), low in ovaries (0.42 +/- 0.46 ng/g), and strikingly high in transitional gonads (2.89 +/- 1.64 ng/g) even at the very beginning of sex inversion. This suggests an important role for this estrogen in the protandrous sex inversion process in the seabass L. calcarifer.