The primary goal of this study was to assess the effect of varying densities on serum reproductive parameters of immature rainbow trout Oncorhynchus mykiss. Experimental trout were maintained in intensive, pen-reared farms for 300 days in fresh water reservoirs. Initial densities were 4.6, 6.6, and 8.6 kg/m~3(40, 60, 80 ind./m~3), indicated as SD1, SD2, SD3, and final densities were 31.1, 40.6, 49.3 kg/m~3, respectively. A summary of the ovarian stages were observed by histological examination. Serum E 2(estradiol), T(testosterone) were evaluated by radioimmunoassay and FSH(follicle-stimulatinghormone), LH(luteinizing-hormone), vitellogenin, 17α,20β-P(17α,20βdihydroxy4-pregnen-3-one) were measured by enzyme-linked immunosorbent assay. Our findings demonstrated that ovarian development were retarded(from stage Ⅲ to stage Ⅳ) at highest rearing density(SD3) after 180 days of intensive culture(over 40.6 kg/m~3). In addition, we observed an inverse relationship between serum reproductive parameters and rearing density. Furthermore, compared to serum reproductive parameters of SD1, E 2, T, FSH, vitellogenin, 17α,20β-P, GSI and LH of two higher density groups decreased firstly and significantly at 60(over 15.9 kg/m~3), 180(over 31.7 kg/m~3), 180(over 40.6 kg/m~3), 240(over 36 kg/m~3), 240(over 36 kg/m~3), 240(over 45 kg/m~3) and 300(over 49.3 kg/m~3) days, respectively. Comparing serum reproductive parameters within the same ovarian development stage of rainbow trout from varying densities revealed that higher population density also led to significantly lower overall serum reproductive parameters. Overall, this study presents the reproductive, endocrinological parameters of juvenile female rainbow trout at high rearing densities and indicates the need for rainbow trout(114.44±5.21 g, 19.69±0.31 cm) that are initially stocked at 6.6 or 8.6 kg/m~3 should be classified and subdivided into lower density after 180 days of farming(not over 31.7 kg/m~3). The primary goal of this study was to assess the effect of varying densities on serum reproductive parameters of immature rainbow trout Oncorhynchus mykiss. Experimental trout were maintained in intensive, pen-reared farms for 300 days in fresh water reservoirs. Initial densities were 4.6, 6.6 , and 8.6 kg / m ~ 3 (40, 60, 80 ind./m~3), indicated as SD1, SD2, SD3, and final densities were 31.1, 40.6, 49.3 kg / m ~ 3, respectively. A summary of Serum E 2 (estradiol), T (testosterone) were evaluated by radioimmunoassay and FSH (follicle-stimulating hormone), LH (luteinizing-hormone), vitellogenin, 17α, 20β-P (17α, 20βdihydroxy4 -pregnen-3-one) were measured by enzyme-linked immunosorbent assay. Our findings demonstrated that ovarian development were retarded (from stage III to stage IV) at highest rearing density (SD3) after 180 days of intensive culture (over 40.6 kg / In addition, we observed an inverse relationship between serum reprodu Furthermore, compared to serum reproductive parameters of SD1, E2, T, FSH, vitellogenin, 17α, 20β-P, GSI and LH of two higher density groups decreased primarily compared to 60 (over 15.9 kg / m 3, 180 (over 31.7 kg / m 3), 180 (over 40.6 kg / m 3), 240 (over 36 kg / m 3), 240 (over 45 kg / m ~ 3) and 300 (over 49.3 kg / m ~ 3) days, respectively. Comparing serum reproductive parameters within the same ovarian development stage of rainbow trout from varying densities that that higher population density also led to significantly lower Overall, this study presents the reproductive, endocrinological parameters of juvenile female rainbow trout at high rearing densities and indicates the need for rainbow trout (114.44 ± 5.21 g, 19.69 ± 0.31 cm) that are initially stocked at 6.6 or 8.6 kg / m ~ 3 should be classified and subdivided into lower density after 180 days of farming (not over 31.7 kg / m ~ 3).