This paper describes a five-component system with ten major internal linkages which may be used as a model for studying information flow in any rural agricultural social system. The major components are production, supply, marketing, research, and extension education. In addition, definitions are offered of the crucial variables affecting efficiency and effectiveness of communication via the linkages. Audience, message, channel, treatment, and impact are described. Based on the system model and the defined variables, simple mathematical formulas are given which illustrate the relationships in impact and efficiency, and which may be used in computer simulation of information flow, or in planning change, in any rural social system. (Author)