1. Diets containing different proportions of skim milk and fat, either beef tallow or soya-bean oil, supplemented with fat-soluble vitamins were given to 2-d-old pigs. Each fat source was used in three diets containing approximately 280, 350 and 420 g fat, and 260, 235 and 210 g protein/kg dry matter respectively..
2. The diets were given as a milk containing 200 g solids/l during a 26 d experiment. The pigs were given the diet at hourly intervals according to a scale based on live weight..
3. Increasing the energy content of the diet, with the accompanying decrease in protein content, had no effect on live-weight gain or feed: gain ratio (g feed dry matter/g live-weight gain) (other than a slight reduction in live-weight gain when beef tallow was used), or on nitrogen retention. However, the fat content of the carcass of the 28-d-old pig was increased, and its protein and ash content were decreased..
4. The apparent digestibilities of the fatty acid ranged from 0.96 to 1.00. The faeces from pigs given the soya-bean-oil diets contained some eicosanoic, docosanoic and tetracosanoic acids, presumably of bacterial origin. In general, palmitic and stearic acids were slightly less well digested than oleic and linoleic acids.