There are well established differences in the digestion characteristics of the carbohydrate sources available for inclusion in rations for dairy cows, but the implications of these differences for milk production remain poorly defined. The experiment reported here was conducted to compare 4 concentrates containing contrasting carbohydrate types, namely sources of readily-fermentable starch (barley), of sugar, of starch that is fermented relatively slowly in the rumen such that some is absorbed as glucose from the small intestine (maize) and of sugar and digestible fibre (citrus pulp). Given the advantages of dietary inclusion of appropriate fats as a means of increasing the energy density of the ration and/or of producing ‘less-saturated’ milk fat, and the likelihood that responses to high-fat diets would be modified by carbohydrate type, the different carbohydrate sources were compared in concentrates containing a high proportion of fat (16% on a fresh weight basis) as calcium salts of long-chain fatty acids.