The response in yields of milk constituents to increases in the amounts of standard dairy concentrate (SC, 200 g crude protein (CP) per kg DM) given to cows consuming grass silage ad libitum were reported recently (Aston et al 1991; Baker et al 1991). A change of level however adjusts the supply of both energy and CP. Previous work also showed .linear responses in silage intake and milk and milk protein yields when CP was increased in a fixed ration of concentrate of constant energy value. In this trial energy and CP levels were varied Independently. The objective was to separate the effects of energy on the performance of lactating cows from those of CP.