Experiments were carried out to investigate the effects of silage additives on ruminai metabolism. Three experimental approaches were adopted; 1) in vivo experiments using sheep; 2) in vitro experiments, and 3) infusion experiments in which silage additives were directly infused into in vitro cultures. In the first two experiments eleven grass silages were used including materials treated with the additives formic acid, acetic acid and formaldehyde, and wilted silages. Both additive treatment and wilting restricted fermentation and resulted in high residual concentrations of water soluble carbohydrates and low concentrations of protein degradation products. The in vivo experiments showed that in all cases restriction of in -silo fermentation increased silage voluntary dry matter intake. Microbiological analysis of rumen contents indicated that silage composition influenced the microbial population of the rumen. Numbers of proteolytic organisms were positively correlated with ruminal ammonia concentration and were depressed when formaldehyde- treated silages were ingested. Silage intake influenced ruminai microbial counts, particularly those of cellulolytic organisms. Development of an in vitro apparatus (The Rumenstat) was an important part of this work. The literature relating to in vitro techniques was reviewed and an apparatus for continuous culture at steady -state was devised, employing established principles and a number of innovations. With the in vitro apparatus, cultures were maintained using the eleven experimental silages as substrates. The results with these cultures were in accord with those in vivo and confirmed that ruminai characteristics were influenced by compositional differences between the experimental silages, and not by the presence of additives as such. The effects of silage additives per se were investigated in the third series of experiments. In vitro cultures were infused with increasing concentrations of formic acid, acetic acid, propionic acid and formaldehyde. The organic acids depressed energy metabolism only at concentrations higher than would result from the ingestion of additive -treated silages. Formaldehyde caused a general reduction in ruminal fermentation at low concentrations. The implications of this work to the future development of in vitro rumen technology and the additive -treatment of silage are discussed.