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Naltrexone—an opioid receptor antagonist, was administered intraperitoneally to sexually mature male and female common carp in the prespawning period, in order to investigate its effects on spontaneous or sGnRH-A-stimulated LH secretion. Naltrexone and sGnRH-A were injected at the same time. The possible involvement of a dopaminergic system in this process was studied in males pre-treated with pimozide (a dopamine receptor antagonist) 12 h before naltrexone and/or sGnRH-A administration. Blood samples for the analysis of carp LH concentrations were taken just before the injections and then after the injections, serial sampling during 24 h was performed. In male carp, naltrexone (500 or 5000 μg kg−1) decreased spontaneous LH release, but there were no effects of naltrexone on sGnRH-A-stimulated LH secretion. In males pre-treated with pimozide, a similar response to naltrexone injection (500 μg kg−1) as in pirnozide non-treated fish, was observed. The highest dose of naltrexone, 5000 μg kg−1, significantly stimulated LH release, in response to sGnRH-A administration in pimozide pre-treated males. In female carp, contrary to males, naltrexone at a dose of 500 μg kg−1, caused significant stimulation of spontaneous LH release. These data indicate that endogenous opioid peptides modify LH secretion in sexually mature carp. In males, they stimulate LH secretion, acting rather on the hypothalamic GnRH system and in females, opioids inhibit LH release by the influence on the dopaminergic system. [Copyright &y& Elsevier]