A mechanistic model describing rumen digestion is presented. The model consists of 22 compartments for classical nutrients, microbes associated with either solids or fluid, and 8 amino-acid compartments. An empirical approach for volatile fatty acids production is used. Sensitivity and behavioural analysis on several parameters demonstrated the need to improve knowledge concerning certain factors. Model results on general criteria are in good agreement with observed data. Validation on 49 experimental lysine and methionine duodenal fluxes were satisfactory. A comparison between observed and predicted values of LysDi and Metdi on several feedstuffs showed good results. This model is a first step in the building of a rumen model suitable for research and application.