This experiment was carried out for the determination of the optimal breeding density of rainbow trout(Oncorhynchus mykiss) and environmental conditions in semi-closed recirculation system. Since under the conditions of various research programmes, comparable trout origins rank differently, it must be supposed that genotype-environment interaction can occur under different stress situation (e. g. different density/PVC rearing unit). Thus, control of test environment of utmost importance both for performance testing and for breeding decisions. The recirculation system consists of 35 single PVC-cylinder with a water volume of 80 litre, respectively. In this experiment water renewal was lOm^3/day and density 75∼400 fries or fingerlings in each cylinder. The concentration of ammonia, nitrite, nitrate, and pH were measured weekly during the experiment . The final fish: water ratio was 1: 18.6(or 150kg live body weight per 2.8m^3 water), Water temperature(9±1˚C), the values of pH(7.3∼7.8), the concentration of ammonia(0.1∼1.Oppm), nitrate(15∼20ppm), total nitrogen(8.7∼9.8ppm), and phosphate(0.3∼2.Oppm) were within the safe range. The feed efficiency was better than any other domestic animal and was 1.46. The mortality was 5.6%, and the optimal stocking density was considered to be 200fish/cylinder It can be concluded that the breeding experiments implemented at the aquacultural test station, except for the beginning phase of each test, have not been affected by environmental factors.