Response surface methodology(RSM)and Central Composite Rotatable Design(CCD)were used to study the combined effects of ammonia(0.02-2.00 mg/L)and density(1-5 ind/10 L)on the specific growth rate(SGR)and antioxidant activities in the liver of juvenile tilapia(Oreochromis niloticus).The objectives were to find out the linear,quadratic or reciprocal effects of ammonia and density on the growth and antioxidant activities in the liver of juvenile tilapia by the regression equation.The results showed that the linear and quadratic effects of ammonia and density on the SGR were significant(P<0.05),SGR increased at the beginning and then decreased with the increased concentrations of the ammonia,and the density effects showed the same trend.There was a significant interactive effect between ammonia and density on growth(P<0.05).Higher SGR was observed when the density was 1-2 ind/10 L and ammonia was 0.02-0.20 mg/L.When concentrations of external ammonia were higher than 0.2 mg/L and density were around 3 indiv/10 L,SGR was also higher.With the increased concentrations of ammonia and density,the content of MDA in the liver increased gradually,while the activities of SOD and CAT increased at the beginning and then decreased.The linear effect of ammonia and density had significant effects on MDA,SOD and CAT(P<0.05),the quadratic effects of ammonia and density had very significant effects on the SOD and CAT(P<0.01).There was a significant interactive effect between ammonia and density on CAT activity.High concentrations of external ammonia and high density inhibited SOD and CAT activities.Model equations of the effects of ammonia and density on SGR,MDA,SOD and CAT were established,with the R² being 0.972 4, 0.913 2,0.938 9 and 0.969 2 respectively(P<0.01),which could be used for prediction.Growth and activities of antioxidant enzymes were more influenced by ammonia than by density.In the cultural practice,it is suggested that the density should be controlled reasonably and the dissolved oxygen be maintained adequate to reduce ammonia in order to improve growth and disease resistance of tilapia.