The effects of gonadotropin-releasing hormone agonist (GnRHa) administration on testicular maturation and apoptosis of male germ cells of Atlantic bluefin tuna (Thunnus thynnus L.) reared in captivity were evaluated. Individuals classified at early and late spermatogenesis were equally present among untreated and GnRHa-treated fish. A lower rate of germ cell apoptosis was observed in GnRHa-treated individuals than in untreated controls. The present study indicates that: (i) wild-caught male Atlantic bluefin tuna reared for 1 to 3 years in captivity did not attain a full testicular maturation; (ii) GnRHa treatment was effective in reducing germ cell apoptosis.