It is well established that the proportion of saturated fatty acids in cows milk fat can be reduced by dietary inclusion of fats containing a large proportion of C18 fatty acids. Regardless of the individual C18 acids added to the diet, because unsaturated C18 fatty acids are biohydrogenated in the rumen it is principally the amount of 18:0 absorbed by the animal which is increased. This leads to increased mammary uptake and desaturation of 18:0 and to the secretion in milk of greater amounts of 18:0 and 18:1 fatty acids. Intramammary synthesis of 6:0-16:0 fatty acids is concurrently reduced. Of the cereals grown in the UK, oats contain relatively large amounts of C18 fatty acids. For this reason we have examined the potential of dietary inclusion of oats as a simple and inexpensive means of manipulating milk fat composition. In a previous experiment in which hay-based diets were given replacement of barley with oats reduced the saturated (6:0-18:0) fatty acids in milk fat from 753 to 619 g/kg and increased the monounsaturated fatty acids from 211 to 344 g/kg milk fat.