Methane formation in the rumen represents a substantial loss of energy to the animal and is also a major source of greenhouse gas emissions from agriculture. Methanogenesis is the main means of disposal of hydrogen during rumen fermentation. The formation of propionate represents an alternative route of hydrogen disposal, providing sufficient propionate precursors are available. Theoretically, adding propionate precursors should stimulate propionate production and decrease methane production. In the present experiment, the effects of two potential precursors of propionate, fumarate and acrylate, on rumen fermentation and methane production were investigated in a rumen simulating fermentor.