Elevation of [K+] in the incubation media to 24.9 mM increased the in vitro uptake of Ca++ by the rat adenohypophysis as measured by 45Ca. There was no change in the extracellular space as measured with tritiated mannitol. The addition to the incubation media of other materials which also provoke the release of the various adenohypophysial hormones had no effect upon Ca++ uptake. The materials added were a crude acid extract of rat hypothalamus-stalk-median eminence, synthetic lysine vasopressin, dibutyryl cyclic AMP, or theophylline. In each case the concentration employed was sufficient to cause release of at least 2 of the adenohypophysial hormones. Thus, in this tissue, only the hormone release produced by elevated K+ is consistent with the “stimulussecretion coupling” model. Perhaps only the presence of Ca++ and not an increased uptake of Ca++ is required for the release of preformed hormone from the adenohypophysis. (Endocrinology89: 766, 1971)