P>The present study aimed to establish some basic characteristics of Atlantic bluefin tuna sperm from captive mature males, treated or untreated by gonadotropin releasing hormones agonist (GnRHa). Intratesticular milt was collected from treated and untreated fish (mean weight +/- SD: 122.9 +/- 29.2 kg, n = 21). There was no significant effect of GnRHa treatment on GSI (1.33 +/- 0.70%, n = 21) or on sperm concentration (3.8 +/- 1.3 x 1010 spermatozoa ml-1, n = 21) estimated by optical density at 260 nm. Similarly, the percentage of motile spermatozoa measured at 30 s post activation: activating medium (AM seawater containing 10 mg ml-1 BSA) was not significantly different between control and GnRHa implanted males. On the other hand, a significantly higher Average Path Velocity (VAP) was assessed for sperm from GnRHa-treated fish, compared to controls. Regardless of hormonal treatment, the percentage of motile sperm decreased after a plateau of 5-6 min post activation, and any forwardly progressing movement ceased after 10-11 min. Linear trajectories of spermatozoa were observed in seawater while tighter circles were assessed when increasing the Ca2+concentration in the AM. The storage capacity at 4 degrees C was significantly lowered when NAM (Non Activating Medium: 50% seawater containing 10 mg ml-1 BSA) was added to sperm at a 1 : 1 dilution. These results demonstrated that treatment with GnRHa had little effects (except on VAP) on sperm characteristics in captive reared Atlantic bluefin tuna. Further investigations are required to improve the knowledge of tuna sperm biology and, in this respect, the comparison between intratesticular and released sperm features should be highly informative.