Recent advances in technology have dramatically increased the resolution with which we may examine many features of biological systems. Intracellular recording and tracer injection techniques allow one to study the function of individual neurons and later characterize the same cells morphologically. In situ hybridization techniques can give us information about messenger RNA levels in single cells. More established techniques such as immunocytochemistry and electron microscopy also provide information at the cellular and even subcellular level. With each of these technological advances we have learned more about the mechanisms underlying cell function. We are also beginning to appreciate the role of heterogeneity among cells in relation to the function of the whole organism. Application of the reverse hemolytic plaque assay to the study of hormone or neurotransmitter secretion should help clarify this role. This technique permits accurate quantitation of hormone secreted from a large number of cells. Thus while cells can be studied individually they can also be categorized into functional subpopulations. As discussed in this chapter, many other techniques may be applied on cells which have already been functionally defined with the plaque assay. This should result in a clearer understanding of the roles of secretagogue binding and internalization, activation of second messenger systems, protein synthesis, and the cytoskeleton in hormone secretion. In the plaque assays described in this chapter individual pituitary cells are isolated in culture free from possible interactive effects coming from other cells. While these interactions are no doubt critical to the understanding of the function of the organism as a whole they can result in totally uninterpretable results. In fact, when we have gained some understanding into the functioning of individual cells it should be possible using the plaque assay to study the interactions among cells in a controlled fashion.