The ionophore monensin is used as a model to examine the modes of action important in manipulating rumen function. Several system modes of action probably result from the basic mode of action of the ionophore modifying the movement of ions across the membranes of rumen microbes. While there are many biological responses reported in the literature for monensin, they can be consolidated into seven categories or system modes of action. The modification of volatile fatty acid production is one widely recognized category of great importance. Modified feed intake should also be considered to be important. The third system mode of action, change in gas production, probably contributes only a limited savings in energy. Modified digestibilities are probably quite variable as a mode of action, but may be a significant factor. The change in protein utilization appears to result from several factors that are occurring simultaneously. Modification of rumen fill and rate of passage may be important in causing some of the previously mentioned system modes of action to occur. A seventh category inclusive of several monensin responses that are more indirect to the rumen, or sporadic in nature, is included. Increased animal production from the use of monensin appears to occur as a result of these several system modes of action, which probably act in concert. It is impossible to accurately assess a quantitative contribution of each of these categories at the present time.