A wide range of compounds has been described which have the potential to improve animal production by manipulating the rumen fermentation. Prominent among these rumen modifiers are the ionophores. Ionophores, such as monensin and tetronasin, improve feed efficiency, partly by increasing the flow of amino-N from the rumen and partly by stimulating the production of propionate in the rumen with an associated reduction in the production of methane (Russell and Strobel, 1988). Recently there has been increasing interest in the use of yeast culture (YC) and other fungal preparation to modify the rumen fermentation. These products have been shown to increase bacterial numbers within the rumen with an associated increase in the breakdown of fibre and supply of microbial protein (Williams and Newbold, 1990). YC has also been reported to increase the production of propionate in the rumen. Little appears to be known about the effect a combination YC and an ionophore would have on the rumen fermentation. This study describes the effects of the ionophores monensin and tetronasin on the fermentation of hay by rumen fluid from sheep fed a basal diet with or without YC.