The quantity of γ- and δ-lactones and of methyl ketones produced when bovine butterfat was heated in the presence of water vapour and the absence of air, and hence the flavour quality of the butterfat, varied widely with the diet of the lactating cows. There was a marked increase in the γ-dodecanolactone potential when the cows were transferred from pasture to a mixture of chopped lucerne hay plus crushed oats but not when they were transferred to lucerne hay only. Feeding crushed sunflower seeds, whose oil was protected against biohydrogenation in the rumen, but not feeding sunflower meal from which the oil had been extracted, raised the γ-dodec-cis-6-enolactone potential, lowered the saturated δ-lactone potential and the methyl ketone potential and simultaneously raised the proportion of linoleic acid in the butterfat at the expense of fatty acids up to and including C16.