We have shown previously (Cooper and Kyriazakis, 1993) that lambs offered a choice between a low energy density (ED) food and a high ED food, do not solely consume the high ED food, as it would have been expected from an optimal foraging perspective. It is proposed that the lambs offered this choice have to balance the benefits of consuming a high ED food (high growth rate) against the metabolic costs incurred, which may be excessive disturbances to the rumen environment, such as a low rumen pH and an increased osmolality (this has been shown to be the case in the diet selection of sheep between concentrates and hay in an experiment by Engku Azahan and Forbes (1992)). The aim of this experiment was to test which of these two rumen conditions have an influence upon diet selections made by sheep offered foods of different ED.