Six lactating dairy cows were used in a three period, part balanced changeover design experiment to investigate the effects of forage digestibility and concentrate composition on the efficiency of nutrient utilization in lactating dairy cows. Six treatments comprising three forage regimens and two concentrate types (starch v. fibre) were examined in a 3 x 2 factorial design. The three forage regimens were high digestibility grass silage offered ad lib. (HA) or restricted to 6.5 kg dry matter/d (HR) and a low digestibility grass silage offered ad lib. (LA). Within each forage regimen animals were offered 10 kg/d of supplements containing either high-starch or high-fibre concentrations. Experimental periods lasted 28 d with a 10 d recording period, during which animal performance, ration digestibility and nitrogen and energy utilization were measured. Respiratory exchange measurements were made over a 72 h period using indirect open-circuit calorimetry. Throughout the experiment, there were no significant forage x concentrate interactions in any of the intake, production or nutrient utilization results. Milk yield was significantly influenced by forage regimen (24.1, 21.7 and 21.9 kg/d for HA, HR and LA respectively) and concentrate type (21.6 and 23.5 kg/d for high-starch and high-fibre respectively). Concentrate type also significantly influenced milk protein concentration (32.8 and 30.9 g/kg for high-starch and high-fibre respectively). Forage regimen significantly influenced the efficiency of utilization of metabolizable energy (ME) for milk production (k1) with values of 0.62, 0.64 and 0.59 for HA, HR and LA respectively. Concentrate type had no significant effect on ME intake, heat production or k1, although animals receiving the high-fibre concentrates synthesized proportionately 0.11 more milk energy per unit of available energy (ME intake--heat production) than those receiving the high-starch concentrates. Interpolation of the values obtained with the two high digestibility forage regimens indicated that at similar ME intakes there was a trend towards a higher k1 with the diet based on high digestibility silage, and this was in line with the higher metabolizability of the overall diet with this silage.