1. Nine experiments, each with one of six sheep with cannulated rumens given a constant diet of dried grass, were made in which oleic, linoleic or linolenic acid was infused into the rumen and energy and lipid metabolism were measured. One experiment was made in which palmitic acid was given. 2. Judged by changes in the composition of isolated fatty acids, the unsaturated fatty acids were hydrogenated in the rumen. An increase in the excretion of lipid in the faeces occurred when the unsaturated acids were given. The heat of combustion of the faeces increased by 12.6±3.0 kcal/100 kcal fatty acid, of which 94% was accounted for by the additional lipid. 3. Methane production fell when the unsaturated fatty acids were infused, the decreases being 13.8±1.6 kcal CH4;/I00 kcal oleic acid, 14.2±1.5 kcal CH4/100 kcal linoIeic acid and 16.4±1.3 kcal CH4/100 kcal Iinolenic acid. The introduction of a double bond into an n-alkyl acid was calculated to reduce methane production by 0.24±0.09 moles/mole double bond. 4. Because the depression of methane production on infusing the fatty acids exceeded the increase in the heat of combustion of the faeces, the metabolizable energy of the fatty acids was 104.1±5.3% of their heat of combustion. 5. The efficiencies with which the fatty acids were used to promote energy retention were 74.6±5.7% for oleic acid, 79.2±2.0 % for linoleic acid and 82.5±3.0% for linolenic acid. These efficiencies agreed with those noted in experiments by others with rats, horses and pigs given glycerides, but were higher than those noted by others when glycerides were added to the diets of ruminants.