Local communication among cells of the anterior pituitary appears to play an important role in the regulation of ACTH secretion. Dissociated pituitary cells were plated as a monolayer at decreasing concentrations of cells (increasing the distance between cells and, thus, decreasing their potential interactions), and ACTH secretion was measured from individual corticotropes using a specific reverse hemolytic plaque assay. There was a critical intercell distance above which significant changes in the number of CRF-responsive corticotropes were observed. Provided that this critical distance was not exceeded the number of secretory corticotropes in response to CRF (10 nM) was relatively constant, thereby defining a fraction of corticotropes that was robustly CRF responsive. In contrast, when this critical distance between cells was exceeded, the number of CRF-responsive corticotropes progressively increased to almost double their original number, thereby defining a second fraction of CRF-responsive corticotropes that was previously repressed. These observations suggest the presence of a paracrine factor that profoundly inhibits CRF-stimulated ACTH secretion from a repressed fraction of corticotropes. Further independent studies confirmed and extended these observations. We identified the cellular source of the inhibitory factor as the robustly CRF-responsive fraction of corticotropes. Pituitary cells were identified by reverse hemolytic plaque assay and then destroyed using a laser photoablation procedure that did not compromise the remaining cells. The pituitary cells were separated by a distance at which the inhibitory factor was fully effective. Destruction of the cellular source of the paracrine inhibition would, therefore, allow secretion from the previously repressed fraction of corticotropes. Accordingly, when robustly CRF-responsive corticotropes were destroyed, a significant number of previously repressed corticotropes appeared in a second assay. Destruction of somatotropes or a cell adjacent to a robustly CRF-responsive corticotrope did not alter the number of CRF-stimulated corticotropes among the remaining cells. We conclude that a paracrine factor liberated by the robustly CRF-responsive corticotropes inhibits ACTH secretion from the repressed fraction of corticotropes. The robustly CRF-responsive corticotropes appear unresponsive to the effects of the factor, and the repressed corticotropes are unlikely to secrete it. A role of this paracrine communication is to hold corticotropes in reserve and, therefore, prevent the severe depletion of hormone. This form of paracrine communication may be a specialized adaption among cells where the physiological setting demands robust secretory responses to multiple stimuli. The experimental paradigms developed here may be extremely useful for 1) screening potential paracrine factors and 2) determining whether the secretion of the paracrine factor is regulated by adrenal or hypothalamic hormones.