Mature male rabbitfish (Siganus guttatus Bloch) received weekly injections of 200 μg of luteinizing hormone-releasing hormone analogue (D-Ala⁶, Pro⁹-LHRH-ethylamide) per kg body weight for 5 consecutive weeks. Mean spermatocrit, or packed sperm volume (27–51%), and mean sperm density (3.2–9.6×10⁶ spermatozoa per kg body weight) decreased significantly 24 h after each injection. The amount of expressible milt (mean: 5.8–11.7 ml per kg) in response to weekly injections of LHRHa increased significantly relative to saline-injected fish (1.0–2.9 ml per kg), but only during the initial 4 weeks of regular hormone treatment. Three weekly injections of LHRHa likewise augmented mean sperm production (29.2–112.5×10⁹ spermatozoa per kg) in rabbitfish. However, no significant enhancement in sperm production by LHRHa-injected fish was observed over the last 2 weeks of hormone injection. These results demonstrate that weekly injection of LHRHa can sustain milt production in mature rabbitfish, although their capacity to produce spermatozoa is limited to only 3 consecutive weeks of regular hormone treatment.