The experiment was designed in a (4x3) factorial arrangement of treatments to investigate the effect of four different mixed protein schedules and three dietary Na/K ratios on Nile tilapia performance. The four protein schedules were achieved by alternating the diet (A ; 31%CP)and diet (B;24°/oCP)as follows (6-days~AX(5days~A/lday-B), (4days-A /2days-B) and (3days-A /3days-B).Meanwhile, sodium chloride was added to study the effect of Na/K ratio of 0.5 ,1.5 and 2 on utilization of dietary protein, amylase and lipase activity and serum thyroid hormone level. Moreover, the interaction between mixed protein schedules and Na/K ratio has been investigated. The results are summarized as follows:
- The optimal protein schedule for tilapia performance was either (5A/lB)or(4A/2B).
- Fish in (5A/1B) or(4A/2B) schedules utilized protein more efficiently than fish fed on higher protein diet continuously (6A>.
- Addition of NaCl to fish diet to achieve Na/k ratio 2 was beneficial and improved fish performance with decreasing inclusion of higher protein diet in protein schedule (3A/3B)
- Increasing Na/K ratio decreased thyroid hormone level in the male and maintained it in contrast with the female
- Raising of Na/K up to 2 decreased lipase activity and ether extract digestibility.
- The lowest feeing cost was recorded for fish fed on on diets with Na/K ratio 2 and fed on on (4A/2B) schedule.