1. Twelve sheep were offered a diet of hay and molassed sugar-beet pulp for 18 weeks, and from the 3rd to the 16th week the sheep were divided into four groups of three; one of each of the groups received daily 170 mg trichloroacetamide, 140 mg chloroform, 30 g linseed oil or no additive.
2. The additives had no significant effect on the apparent digestibility of the dry matter, organic matter, nitrogen or energy of the diet.
3. When either trichloroacetamide or chloroform was added to the diet, methane production was almost totally suppressed and some hydrogen was produced but, although these changes persisted with chloroform, they were much reduced after 28 d of trichloroacetamide administration. When linseed oil was added to the diet the methane production was reduced by almost 15%.
4. Each additive caused a significant reduction in the concentration of total volatile fatty acids in the rumen liquor and in the molar proportion of acetic acid in the total volatile fatty acids, with a corresponding increase in the molar proportion of propionic acid. The effect was greatest immediately after the introduction of the compound into the diet but it diminished and appeared to attain a stable value within the experimental period of 98 d.
5. All the effects disappeared within 14 d of the withdrawal of the compounds from the diet.
6. It is suggested that these effects may be the result of selective effects on the rumen microflora and that further work is needed to find an additive whose effects are persistent.