The effects of overfeeding by gastric intubation on the body composition and energy metabolism of adult female rats were studied in three experiments. 2. In Expt 1 there were significant (P less than 0.001) linear increases in carcass dry matter, fat and energy during a 10 d period as metabolizable energy (ME) intake was increased from 160 to 300 kJ/d. 3. In Expt 2 rats were fed to maintain weight (130 kJ/d) or given approximately 270 kJ/d for 120 d. Measurements of fed and fasting heat production (FHP) were made at intervals. FHP (kJ/d per kg metabolic weight (W0.75)) decreased by 15% over the 120 d period on both treatments. The mean carcass weight of the overfed rats increased from 216 to 465 g. over 90% of the increase being due to fat. 4. In Expt 3 rats were fed to maintain weight (137 kJ/d) or given approximately 300 kJ/d for 6, 12, 18, 24 or 30 d. There were significant linear increases (P less than 0.001) with time in carcass weight, dry matter, fat and energy. FHP, measured before slaughter, increased from 118 to 160 kJ/d but remained constant at 334 kJ/d per kg W0.75. 5. In all three experiments there were significant (P less than 0.01) increases in carcass crude protein (nitrogen x 6.25) in response to overfeeding. 6. The efficiency of utilization of energy for production (Expt 1) or for maintenance and production (Expts 2 and 3) averaged 0.92, 0.86, 0.88 respectively. 7. It is concluded that FHP per kg W0.75 may be regarded as constant over a wide range of body compositions in adult rats made obese by gastric intubation, and that energy utilization conforms to classical concepts.
