The effects of an intra-ruminal infusion of a mixture of methionine (7.0 g day-l) and sodium sulphate (6.7 g day-l) on the digestion of grass silage were examined in four sheep fitted with cannulas in the rumen and proximal duodenum. There were no significant (P z 0.05) effects of the infusion on the ruminal digestion of OM, cellulose or N, or on the concentrations of total or individual VFA in the rumen fluid though the numbers of protozoa in the rumen were reduced (P < 0.05). The amounts of total methionine or of bacterial nitrogen reaching the duodenum were not significantly (P> 0.05) altered by the infusion and rates of bacterial synthesis were 21 and 23 g N kg-1 OM (s.e.d. 3.7, P>O.O5) apparently digested in the rumen for the control and infusion treatments respectively. The infusion resulted in a significant ( P < 0.01) increase of 21 % in the amount of total long-chain fatty acids at the duodenum. This reflected an increased flow of all fatty acids of chain-length 14:O and above though more than 80% the response was accounted for by the increased flow of 18:O and 16:0 acids. It is concluded that the infused supplement stimulated the synthesis of microbial lipid in the rumen.