1. Eight groups each of four castrated male pigs, 25–30 kg initial body-weight, were kept for periods of 3 weeks in a calorimeter equipped as a pig pen and maintained at either 8° or 20°. At each temperature two feeding levels (g food/kg body-weight per d) were used, 45 and 52 at 8°, and 39 and 45 at 20°. Metabolizable energy, heat loss and nitrogen balance were measured.
2. Heat loss was higher at 8° than at 20° and was independent of plane of nutrition, whereas at 20° the higher heat loss occurred at the higher plane of nutrition. Energy retention depended on both temperature and feeding level, and was highest at the 52 g feeding level at 8°.
3. N retention was not influenced by environmental temperature but varied with plane of nutrition (correlation coefficient = 0·94), the increase being 9·98 (± 0·8) mg N per g food increase. The correlation coefficient between N retention and body-weight gain was also 0·94; body-weight gain was correlated with N retention rather than with fat deposition. Fat gain was reduced at the lower feeding levels and at the lower environmental temperature at the feeding level of 45 g/kg.
4. The partial efficiency of energy retention at 20° was 66·5%. From this efficiency the maintenance requirement (at zero energy retention) at 20° was calculated to be 418 kJ/kg0·75. At 8° the partial efficiency of energy retention was 99·4%.