I. Introduction Because of the intimidating complexity of homeostatic regulation, the modern researcher in the life sciences is generally obliged to consider physiological phenomena in the most simple, and unfortunately often the most simplistic, terms. While this approach may be initially necessary in certain areas to maintain some semblance of order and sanity, it can become stultifying if allowed to propagate too long. Every physiological action must be recognized ultimately as occurring only in response to a persistently dynamic situation involving any number of influential factors. This may be likened to a strategic battle in which opposing forces are warring on constantly shifting terrain, aided or hindered by an army of diversified hand-picked specialists, with an apparent outcome which can be instantaneously reversed by the sudden appearance of auxiliary support and reinforcements. Many excellent review articles and books have been published during the last decade dealing with the mechanism of action of steroid hormones. From its inception in the early 1960's, receptorology has advanced to a much more sophisticated level as a result of extensive refinements in the methods for measurement of receptors; the detection and delineation of discrete systems in which receptor fluctuations can be assessed in the relative absence of extenuating influences; an awareness of the necessity for concomitant quantification of total cell receptor content and distribution; wide scale independent investigation of a multitude of tissues responding in different ways to different classes of steroid hormones; application of the most advanced and current concepts of molecular biology to the area of steroid-receptor interactions; and continued progress in the pursuit of receptor purification and characterization. Often as a by-product of such studies, an assortment of factors, both endoge-nous and exogenously introduced, have been discovered to influence receptor activity to varying degrees and often in conflicting directions. In the present review, an attempt will be made to analyze the effects of some of these factors within arbitrarily-chosen groupings, in order to assimilate the current status of knowledge in this area. It is felt that such a compilation may be of value in focusing attention on the functional activity of the receptor as it exists within the living cell, rather than simply in an isolated in vitro tissue preparation. It is not within the scope of this discussion to present a comprehensive analysis of all the work done in this field; therefore, most of the stress will be placed upon recent observations as they pertain to our understanding of the basic concepts of receptor regulation.