Feeding fat to increase metabolisable energy content of lactation rations for dairy cows is now a common practice. It generally increases milk yield but decreases milk protein concentration. In North America whole oil seeds (cotton seeds, soya beans, sunflower seeds, etc.) containing high levels of linoleic acid are often fed as a fat source in a total mixed ration (TMR). Rape seed has also been fed and provides higher levels of oleic acid, but rape seed must be processed to be digested. Historically dietary fats for dairy cows have been rendered inert in the rumen to minimise negative effects on fibre digestion. In North America it is now recognised that unsaturated fats fed as whole oil seeds in a TMR do not have deleterious effects on digestion or intake, but these rations typically do not contain grass. Therefore our objective was to determine the intake and lactation response to feeding whole oil seeds (cotton seed and rape seed) in a grass silage-based ration and compare the response to that obtained using a rumen protected fat source as a positive control.