To identify the separate and combined effects of energy and crude protein (CP) from concentrates on responses to concentrate feeding, 42 multiparous Friesian cows were given fixed amounts of concentrates with primary growth grass silage (194 g CP per kg dry matter (DM)) ad libitum for weeks 4 to 22 of lactation. A basal treatment of 3 kg DM per day of a standard concentrate (197 g CP per kg DM) was compared with 6 kg DM per day of the same concentrate (doubling energy and CP intake), 6 kg DM per day of a low-protein concentrate (106 g CP per kg DM) (doubling energy intake alone), or 3 kg/day of a high-protein concentrate (383 g CP per kg DM) (doubling CP intake alone). Results are presented in terms of main effects as interactions were not significant except for energy apparent digestibility. Silage intake was reduced by energy but increased by CP; in consequence extra energy from concentrates increased digestible energy (DE) intake but reduced CP intake and concentration whereas extra CP from concentrates increased not only DE intake but also CP intake and concentration. The ratio of the responses to CP compared with the responses to energy were 1·7 for milk yield, 0·9 for fat yield, 3·2 for protein yield and 1·4 for lactose yield. Energy reduced milk protein concentration and tended to increase milk fat concentration whereas CP had the opposite effect. The efficiency of converting food nitrogen to milk nitrogen was unaffected by extra CP but was increased, from 0·195 to 0·229, by extra energy. The poor response in milk protein yield and the reduction in its concentration with extra energy reflected the lower CP intake due to the reduced silage consumption. The experiment emphasizes the importance of concentrate CP supply in stimulating silage intake and yields of all milk constituents for diets containing fairly small amounts of concentrates.