Three forms of gonadotropin-releasing hormone (GnRH) have been recently identified in the brain of gilthead seabream (Sparus aurata): salmon GnRH (sGnRH), chicken GnRH-II (cGnRH-II), and a novel form, Ser8-mammalian GnRH, named seabream GnRH (sbGnRH). sbGnRH is the most abundant form in the pituitaries of sexually mature seabream during the spawning season. The present study investigated the gonadotropin-releasing activities of the three native forms of GnRH found in seabream brains, as well as of two structural analogs of sbGnRH. All native forms of GnRH stimulated gonadotropin-II (GtH-II) secretion in preovulatory female seabream. cGnRH-II was found to be 7 to 8 times more potent than sbGnRH and 2 times more potent than sGnRH in inducing GtH-II release. sGnRH was found to be 3.5 to 5 times more potent than sbGnRH in inducing GtH-II secretion. These data demonstrate that cGnRH-II, which is not present in pituitaries of sexually mature seabream, is the most potent GtH-II releaser, whereas sbGnRH, 500 times more abundant than sGnRH in the pituitary of maturing fish, is the least potent. The lower potency of sbGnRH may suggest faster enzymatic breakdown, more rapid clearance from the circulation, or a lower binding affinity to the pituitary GnRH receptor. The lower bioactivity of sbGnRH may be compensated for by its high levels in the pituitary. The two analogs of sbGnRH, [D-Nal(2)6,Pro9-NEt]-sbGnRH and [D-Arg6,Pro9-NEt]-sbGnRH, were equipotent to each other and 5 times more potent than sbGnRH in inducing GtH-II release in preovulatory seabream. However, they were 5 to 6 times less active than the analog of mammalian GnRH, [D-Ala6,Pro9-NEt]-mGnRH. Strategies for designing superactive analogs of sbGnRH are discussed.