The high- and low-growth lines of chickens have been developed from a singlefounder population by divergent selection for body weight at 56 days of age formore than 40 generations. The two lines show a ninefold difference in bodyweight at selection age and several interesting correlated selection responsessuch as altered body composition and metabolic differences. We have generated areciprocal intercross comprising >800 F2 birds. In a previous study, we reportedthe detection of 13 quantitative trait loci (QTLs) affecting growth. Here wereport QTLs for body composition (fat deposition, muscle development), weight ofinternal organs, and metabolic traits (plasma concentrations of glucose,insulin, cholesterol, glucagon, triglycerides, and IGF-I). Most of the QTLs withconvincing statistical support mapped in the vicinity of growth QTLs. One of themost interesting observations was that the type of reciprocal cross had highlysignificant effects on body weight at hatch and on plasma concentrations ofglucose, cholesterol, insulin, and IGF-I, but it had no significant effect onbody weight at 56 days of age. The reciprocal cross explained between 15 and 35%of the phenotypic variance for weight at hatch and for plasma concentrations ofglucose and insulin. The observed pattern indicated that these effects werecaused by maternal effects or by genetic differences in mitochondrial DNA.