The current energy (E) feeding standards (AFRC, 1993) have the objective of providing accurate feeding of dairy cows when there is either zero tissue E retention or a given tissue E change. Such approaches are of limited practical value in the real world in which we must be able to predict optimum feeding levels and strategies for animals of differing milk yield potential producing in a range of physical and economic environments. In the latter context the key economic factor is how the animal responds to additional increments of feed. This is primarily driven by how the animal partitions that additional E between milk output and body tissue gain. The objective of this experiment was to use calorimetric techniques to explore the impact of level of metabolisable E (ME) intake (MEI) on milk E output (E1) and tissue E gain (E g ) and hence partitioning of increments of MEI between milk and tissues.