The present energy rationing system used in the U.K., the metabolisable energy (ME) system (AFRC, 1990), is an empirical model incorporating a factorial approach to estimate the requirements of animals. Axiomatically, when this approach is adopted each component of the system must be accurately determined. Recent studies at this Institute using regression techniques have indicated that ME requirements for maintenance (MEm) are greater than that predicted using the equations of AFRC (1990), while the efficiency of use of ME for lactation (kl) is in line with the latter predictions (Kirkland and Gordon, 1999). There is a paucity of recent data relating to the efficiency of use of ME for tissue gain during lactation (kg), and the efficiency of use of body tissue for milk production (krl). The objective of the present study was to use regression techniques with data obtained from several recent calorimetric trials at this Institute to predict the MEm and efficiency factors of lactating dairy cows offered different diet types.