Increasing concern over the role of greenhouse gases in global warming has led to a renewed interest in models for predicting methane production in ruminants. We have previously shown that data generated in sheep may be of limited use in predicting the effect of diet on absolute methane production by cattle (Newbold et al., 1995). However, sheep may still be a viable model for evaluating methods of reducing methane production by cattle. As dietary lipids may inhibit ruminal methanogenesis (Moss, 1993), we have now compared the effect of coconut and linseed oil on methane production by both sheep and cattle.