The plasma concentrations of testosterone (T) of cultured jundia Rhamdia quelen increased progressively during spermatogenesis and reached a peak at the beginning of spermiation (e.g. fluidization of the sperm and production of milt). Plasma peaks of T occurred simultaneously to the highest production of T by the testis, however, both plasma concentrations an testis production of T decreased progressively during spermiation. Concentrations of 11-ketotestosterone(11-KT) peaked c. 1 month after the peak of T, concomitant with enhanced production of 11-KT by the testis. The concentrations of 11-KT during the first cycle were extremely high (>1 mug ml(-1)), but less during the second and third cycles, suggesting that this steroid is most important during the pubertal phase in male jundia. Plasma concentrations of both 17-hdroxy-4-pregene-3,20-dione (17-P) and 17,20beta-dihydroxy-4-pregen-3-one (17,20beta-P) Were significantly higher during spermiation than at an other times, and showed little variation. The pattern of steroid changes was cyclical and consistent with their proposed roles in male fish reproduction. (C) 2002 Published by Elsevier Science on behalf of The Fisheries Society of the British Isles.613751763