The hypothesis advanced in this article requires further validation. Undoubtedly it will require modification as our knowledge of biochemical control increases. Nevertheless, it should prove useful in focusing attention on the apparent similarity in the response of a large number of specific cell types to particular stimuli. Emphasis has been placed on a few common and apparently key elements in these responses. It is recognized that other factors are undoubtedly involved. Specifically, the changes in membrane potentials indicate the likelihood of widespread changes in the properties of the cell membrane, for example, changes in Na + and K + transport and distribution. These aspects of cellular responses may eventually prove to be of equal or greater importance than those common aspects of the system already identified.