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The role of gonadotropin‐releasing hormone (GnRH) in spermiogenesis and spermiation is not well understood. This study examined the effects of different modes of administration of a GnRH analog (GnRHa; [D‐Ala6,Pro9NEt]‐mGnRH) on spermiation in the sea bass, Dicentrarchus labrax. Groups of sea bass received either GnRHa injection in saline (I; 25 μg/kg body weight [BW] or one of three types of GnRHa sustained release polymeric device: a fast‐releasing implant (ethylene‐vinyl acetate copolymer [EVAc]; 100 μg/kg BW), a slower‐releasing implant (ethylene‐vinyl acetate copolymer [EVSL]; 100 μg/kg BW), or biodegradable microspheres (M; 50 μg/kg BW). Total expressible milt was collected and assessed for volume, motility, and counts at various intervals for 44 days. Untreated males produced a total of 9.4 ± 3.4 ml/kg BW (mean ± SEM) of milt over the 44‐day experimental period, with milt production ending by day 28. All GnRHa treatments stimulated a significant increase in total milt volume (ml/kg BW) peaking by day 2 (I: 11.8 ± 1.8) or day 7 (EVAc: 20.0 ± 1.5; M: 26.8 ± 2.7; EVSL: 27.9 ± 4.3) posttreatment. While milt volumes in injected males returned to control values by day 14, all groups treated with sustained GnRHa delivery systems maintained significantly elevated milt volumes for 21 days (EVAc group) or 35 days (EVSL and M groups). Sperm motility was consistently good to excellent (70–100% vigorously active) in all groups when expressible milt volume was above 1.0 ml, and there were no significant differences in sperm counts (averaging 5.5 ± 0.1 × 109 spermatozoa/ml). These data show that sustained administration of GnRHa significantly increases and prolongs spermiation in the sea bass without altering sperm concentration or quality. © 1996 Wiley‐Liss, Inc.