There has been a great deal of work carried out on the control of voluntary intake by sheep but little on the selection of foods. In view of the fact that grazing ruminants usually have a choice of plant materials there is a need to understand what controls their selection. Introduction of salts of short chain fatty acids into the rumen depresses intake of a single feed (Balle and Forbes, 1974; Forbes, 1986); sodium chloride has also been shown to depress food intake (Ternouth and Beattle, 1971; Carter and Grovum, 1988; Grovum and Blgnell, 1989) and it is now clear that the osmolality of rumen contents has a marked effect on intake. There have been several observations on food selection by sheep (e.g. Cropper et al. 1985; Gllmp, 1971; Gordon and Tribe, 1951) but none of these have Involved manipulations of short chain fatty acids or osmolality in the rumen.