The aim of this longitudinal study was to investigate the effects of an isocaloric diet different in crude protein content (15% crude protein, LP versus 20% crude protein, MP) on the performance, carcass composition and on plasma hormone levels of heterozygous Kabir naked-neck (NA/na+) and normal-feathered control (na+/na+) broiler chickens. Body weight of na+/na+ chickens averaged higher compared to NA/na+ chickens. Decreasing the dietary protein content depressed live body weight in body lines to the same extent as no significant interaction between feed composition and line was observed. Cumulative feed intake was higher for na+/na+ chickens but was not influenced by dietary composition. Cumulative feed conversion as well as abdominal fat content was not different between lines but was higher for chickens of both lines when reared on the LP diet. Chemical analysis of 56-day-old chickens revealed a higher abdominal fat content of NA/na+ chickens but no significant line differences for dry matter, total fat and protein content. Feeding the LP diet increased abdominal fat, total fat and dry matter content; an effect which was more pronounced for female than for male chickens of both lines. Protein content was higher for males than for females and was decreased when chickens were reared on the LP diet. Feeding the LP diet significantly decreased plasma T4 and IGF-I levels whereas plasma T3 and GH levels were increased. No significant differences between tines in hormonal status were observed.