A method to quantify the spermiation response of mature rabbitfish to hormonal therapy is described. Spermatocrit or packed sperm volume was measured after milt loaded in non-heparinized microhematocrit capillary tubes was centrifuged for 45 min at 15 000 × g. Sperm density was estimated by a standard hemacytometer method. However, sperm density can be predicted from spermatocrit since their relationship is highly and positively correlated (Y = 29.5 + 2.95 Xr = 0.83; P < 0.01). Milt production by mature rabbitfish was highest (4.9 ml per kg body weight) at 24 h after injection of 20 μg of luteinizing hormone-releasing hormone analogue (D-Ala⁶, Pro⁹-LHRH-ethylamide) per kg body weight and coincided with low spermatocrit (68%) and sperm density (14.1 × 10⁶ spermatozoa per μl milt) levels. These results demonstrate that spermiation in mature rabbitfish can be reliably assessed by a spermatocrit method and that LHRHa can effectively stimulate spermiation in this species.