Studies of the role of the microtubule-microfilamentous system in insulin secretion have been widened by continuing experimentation and analysis to provide a comprehensive working hypothesis which embraces ideas of the way in which the polymerization of microtubules and microfilaments may be regulated and how these cytoskeletal components may act together to enhance the process of granule movement. It is also possible to speculate about, but not yet to demonstrate, the way in which the activities of this effector system could be regulated by calcium and by cyclic AMP, which are essentially involved in the regulation of rates of secretion.