1. The fermentation in the presence of four known methane inhibitors was investigated in rumen contents from sheep given molasses ad lib. and showing two different fermentation patterns.
2. The average hydrogen recoveries obtained with the high butyrate and high propionate patterns were 93 ± 2 and 95 ± 2% respectively.
3. Sodium sulphite, chloral hydrate and a hemiacetal of chloral and starch (HCS) all inhibited methane production, and were associated with an accumulation of hydrogen and lactate in rumen contents showing the high butyrate fermentation pattern. Propionate production was slightly stimulated.
4. In rumen contents showing the high propionate fermentation pattern, linseed-oil hydrolysate depressed methane production at high levels only, increased hydrogen and lactate production but depressed all other products, and HCS depressed methane production and slightly increased propionate production.
5. Low hydrogen recoveries in the presence of the inhibitors were probably associated with the utilization of metabolic hydrogen in reactions not accounted for in the scheme under investigation.