1. A new open-circuit respiration unit consisting of 6 respiration chambers, gas analysis unit and data-acquisition system is briefly described. 2. Energy and protein metabolism in broiler lines selected for improved food efficiency (FC) or for growth rate (GL) were measured weekly from 3 to 6 weeks of age. 3. Gross and apparent metabolisable energy intake per kg W0.75 was on average higher for GL than for FC chickens without differences in metabolisability. Fed and fasted heat production per kg W0.75 did not differ between the lines. FC chickens retained less energy per kg W0.75 than GL chickens. 4. FC chickens deposited much less of the retained energy as fat than their GL counterparts and also showed greater protein conversion efficiency. The leaner composition of the body weight gain in FC chickens was confirmed by the estimated lower fat deposition per kg W0.75 and by the lower fat: protein ratio.

The depressing effect of high rearing temperature (30°C) on body weights of heterozygous naked-neck broiler chickens was apparently less pronounced than in control broiler chickens but no effect of genotype, rearing temperature or interaction on abdominal fat pad content was found. Both genotype and rearing temperature affected plasma tri-iodothyronine but not growth hormone or insulin-like growth factor 1 levels in plasma. Rearing temperature of 20°C or 30°C had effect neither on the slope of the thermoregulatory heat production curves nor on the lower critical temperature in either genotypes except at 4 weeks of age.