1. Sixteen experiments were made with eight sheep to determine the effects of various compounds on methane production, digestion and metabolism. 2. Two experiments with two sheep given stearic acid incorporated in the diet showed that it depressed CH4 production by 2.7 moles/mole stearic acid. The digestion of the basal diet was also depressed, the apparent digestibility of dietary cellulose falling from 61.7 to 55.0%. 3. Three experiments with two sheep given liquid paraffin (mainly the C18 hydrocarbon) by infusion into the rumen showed that it had no effect on CH4 production and that it increased the excretion of fatty acids in the faeces and the non-lipid energy of the faeces. 4. Oleyl alcohol infused into the rumen of one sheep had no effect on CH4 production. It was excreted unchanged in large amounts in the faeces and increased the faecal excretion of fatty acids. 5. A preparation of sulphated C18 and Cla alcohols infused into the rumen of one sheep reduced CH4 production by 11.4 moles/mole sulphated alcohol. The preparation also caused a large increase in the excretion of non-lipid calories in the faeces and in the heat production of the animal. A larger amount of sulphated alcohols, given by infusion into the rumen of another sheep, reduced CH4 production to 41% of the initial values in 5 days, but caused refusal of food. 6. Two experiments were made with two sheep in which lauric acid was given by infusion into the rumen: Food refusals occurred within 48 h of commencing the infusion. 7. Sodium lauryl sulphate given to two sheep by infusion into the rumen reduced CHI production by 8.4 moles/mole. It greatly increased the faecal loss of non-lipid energy and also increased the heat production of the animals. When two further sheep were given sodium lauryl sylphate incorporated in the diet, cellulose digestion was depressed. Evidence of hydrolysis of the sodium lauryl sulphate was obtained. 8. Lauryl alcohol had no effect on the CH4 production of one sheep, but increased the faecal excretion of lipid. Inorganic sulphate had no effect on metabolism in an experiment with a further sheep. 9. The results, together with previous work with oleic, linoleic and linolenic acid (Czerkawski, Rlaxter & Wainman, 1966a), suggest that depression of CH4 production is a function of molecules with both polar and non-polar activities, that is with surface-active properties, and that such compounds when given to ruminants have a greater affect on the CH4-producing organisms than on organisms concerned in cellulose digestion.