1. 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 < 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 < 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 < 0·01) increases in carcass crude protein (nitrogen × 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.