The effects of diets, regulated in respect of both dietary cation-anion difference and dietary Na(+)/K(+) ratio (dietary CAD-Na/K level), on growth, feed efficiency and serum mineral concentration in rainbow trout were investigated. Five dietary CAD-Na/K levels (101-0.4; 352-1.32; 560-2.15; 703-2.63; 981-3.53 meq kg(-1) mol/mol) were used in the experiment. A total of 500 fish, 2 groups for each treatment consisting of 50 fish with a mean initial body weight of 87 +/- 1.75 g were used in the trial. The experiment lasted fifteen weeks. Increasing both dietary CAD level and dietary Na/K ratio has significant positive effects on growth performance and feed efficiency in. rainbow trout. It was concluded that weight gain, specific growth rate, thermal-unit growth coefficient and protein efficiency rate increased linearly, feed conversion rate decreased linearly and daily feed intake was not influenced with increasing dietary CAD-Na/K level between 101-0.4 and 981-3.53 (meq kg(-1) mol/mol). Na, K, Cl and CAD level in the serum remained constant among groups. The dietary CAD-Na/K levels between 560-2.15 and 981-3.53 (meq kg(-1) mol/mol) produced the best growth for rainbow trout. Use of Na(2)CO(3) and NaHCO(3) as a cationic supplement and Na source was appropriate for making diet cationic and increasing the CAD level and Na/K ratio of diet.