1. Two inhibitors of rumen methane production were given to sheep and their effects were measured using an in vitro technique.
2. The substances used were trichloroethyl pivalate (TCE-P) and trichloroethyl adipate (TCE-A). In one experiment with two sheep TCE-P was injected into the rumen and in another experiment with two sheep the same inhibitor was mixed with the food just before feeding. In another experiment, TCE-A was given to two sheep at two dose levels, and in an experiment with four sheep two levels of inhibitor and two levels of diet were used.
3. In general the inhibition of methane production was greater with samples of rumen contents taken after feeding than with those taken before feeding. When sheep were given 120–300 mg TCE-P/d the inhibition of methane production ranged from 21 to 81%. When sheep given maintenance rations were given 125–300 mg TCE-A/d there was 28–90% inhibition of methane production. When sheep were given twice maintenance rations 150 mg TCE-A/d gave no inhibition and 300 mg TCE-A/d gave very low inhibition of methane production (about 16%).
4. In most experiments there was a significant increase in the propionic acid:acetic acid concentration ratio in the rumen when methane production was inhibited.
5. The possible practical use of inhibitors is discussed.