Seventy-five female red-spotted grouper ( Epinephelus akaara
) of 2-3 years old were divided into three groups [control group ?, high (H) and low dose group (L), 25 fish per group] . Each group was treated as follows: high dose group [300 μg luteinizing-hormone releasing-hormone analogue (LHRH-A)/kg body weight], low dose group (100 μg LHRH-A/kg body weight) and control (no LHRH-A). Blood samples were collected on day 0, 10, 20, 30 and 40 after implantation and serum estradiol-17β (E2) and testosterone (T) were determined by radioimmunoassay (RIA). The fish were killed and the gonads were removed and weighed for calculation of the gonadosomatic index (GSI). The microstructure and ultrastructure of ovary, liver and pituitary were examined by both light microscope and transmission electron microscope. The results showed that the ovulation rate in the LHRH-A implantation group was significantly higher than that in the control group; that the ovulation rate of the high dose group was significantly higher than that in low dose group. The GSI was 1.055% and migrated nucleus was observed on day 10 in high dose group, ovary developed slowly in low dose group. The GSI reached a high of 1.858% and a low of 0.987% at day 20 in the mature ovary in the LHRH-A implanted group. Serum T and E2
increased significantly after the LHRH-A implantation at day 10 and declined significantly at day 20. Serum T was significantly lower than the control group. Hypertrophied GTH cells, distinct degranulation and marked dilatation of cisternase of the rough endoplasmic reticulum suggest the release of hormone from the GTH cells. These results suggested that sustained administration of LHRH-A significantly induced ovarian maturation and ovulation in the red-spotted grouper. Sex steroid hormone is correlated with vitellogenesis, not ovulation.