1. Two dairy cows were maintained by intragastric infusion of volatile fatty acids and casein. Except when fasting, the casein-nitrogen was held constant, while total gross energy supply was varied from zero during fasting to 650 kJ/kg body-weight (W)0·75.
2. One cow was estimated to attain zero N balance at an energy intake of 255 kJ/kg W0·75 and the other at 307 kJ/kg W0·75, which was calculated to be substantially below the estimated energy required for zero energy balance.
3. When the cows were later given an N-free infusion for a period preceding the trial, N balance occurred at 98 kJ/kg W0·75 for one cow and 115 kJ/kg W0·75 for the other.
4. Four steers were similarly nourished by intragastric infusion and the energy nutrient increased from 0 at fasting to 450 kJ/kg W0·75. The protein was held constant at 1 g N/kg W0·75 except at fasting. The energy level at which N balance occurred was 154 (SE 38) kJ/kg W0·75 or approximately equal to the energy content of the protein. The practical implications of these findings are discussed.