Whole oil seeds represent an alternative to many commercial rumen-protected fat sources as energy supplements in rations for lactating dairy cows. Rumen protection reduces the potential for negative effects of unsaturated fatty acids on fibre digestion, but the structure of many whole oil seeds are thought to reduce the reactivity of their fat in the rumen. Cotton seed is often imported for inclusion in UK dairy rations, but rape seed represents a home grown oil seed which has potential as an economical fat and protein source in UK dairy rations. However, the seed must be crushed or chemically treated to be digested effectively and crushing may liberate oil to the extent that rumen digestion is altered. In a 20 week lactation study, supplemental fat from rumen-protected fat, cotton seed and rape seed fed at 25 g/kg dry matter (DM) in a grass-silage based total mixed ration (TMR) increased milk yield to a similar extent. However, DM intake was reduced by cotton seed and milk protein was reduced by rumen-protected fat (Reynolds et al., 1998). These responses may reflect alterations in digestive function, thus the objective of the present study, conducted simultaneously to the lactation study, was to evaluate the effects of the same diets on rumen, postrumen and total digestion in lactating dairy cows.