SUMMARYIncubation of rumen fluid for short periods (5 hours) with various sulfur compounds resulted in the accumulation of methanethiol and dimethyl sulfide in the headspace above the fermentation mixture. Suitable substrates included methionine, S-methyl cysteine and dimethylacetothetin among others. Rumen methanethiol appears to arise from direct dethiomethylation of thiomethyl compounds. Some, if not all, of the methionine dethiomethylase activity of rumen fluid requires pyridoxal phosphate as a cofactor. Such a requirement would eliminate the possibility that the dethiomethylation reaction was preceded by deamination as described for certain soil organisms.Benzyl viologen was found to inhibit the production of methane and methanethiol in the presence of methionine and S-methyl cysteine but to have no consistent effect on the production of dimethyl sulfide from these substrates. On the other hand, the dye stimulated the production of methanethiol and dimethyl sulfide from dimethylacetothetin. Carbon tetrachloride e~fectively prevented the formation of dimethyl sulfide but not methanethiol when the sulfur amino acids were used as substrates, but stimulated dimethyl sulfide and methanethiol formation from dimethylacetothetin. Methyl methionine sulfonium chloride gave little or no methanethiol or dimethyl sulfide when incubated with rumei1 fluid. Results are consistent with the concept that rumen methanethiol and dimethyl sulfide can arise from sulfur amino acids that are constituents of the normal ruminant diet.

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