Abstract: The present study was carried out to investigate the influence of energy and protein contents of the diet on growth performance and to evaluate further interactions of dietary manipulation and genotype on endocrine parameters in broiler chickens. 1200 day-old male and female broiler chicks from 2 commercial broiler strains (Hybro and Ross) were fed ad libitum from day 1 onwards by 4 different diets either with a high or low energy combined with a high or low crude protein content. Body weight and feed intake were measured weekly. Venous blood samples were taken (10 birds/sex /line/diet group) weekly from day 14 on, for plasma Growth Hormone (GH), Triiodotyronine (T3) and Thyroxin (T4) content. Relative growth rate was higher in broiler chickens which fed the low energy diet compared to high energy diet during the first week of age, while from week 1 on, a reverse effect of energy on relative growth rate was found. The effect of energy and protein on feed intake was significant (p<0.05), while no interaction between energy and protein was found. Within high energy diet, birds fed low protein diet showed a tendency for higher T3 levels, while on low energy diet, broiler chicks fed high protein level showed a tendency for higher plasma T3 concentrations, probably explaining the presence of a significant energy×protein interaction. Protein content of the diet, significantly (p<0.05) affected plasma T4 levels while energy content had no effect. The higher GH as well as T3 levels in the younger chicks may be an indicator of higher requirement of these hormones for protein synthesis during the fast growth phase. It can be concluded that higher protein content of the feed could result in lower T3, higher T4 and higher plasma GH levels. Possible causal mechanisms underlying this phenomenon are discussed.