W e have previously reported that in hypothyroid turkeys the number of beta-adrenergic receptors in intact erythrocytes is reduced by ∼50% without any changes in the affinity of the receptor for the agonist, isoproterenol. In view of the physiological action of the catecholamines to stimulate bidirectional ion fluxes in these cells, we have now examined the possibility that the decrease in beta receptor number might be associated with concomitant changes in catecholamine-dependent potassium ion transport. Hypothyroid turkey erythrocytes display decreased sensitivity to isoproterenol-stimulated potassium influx. Half-maximal stimulation of potassium influx occurs at 9.2±1.7 nM in hypothyroid cells as opposed to only 3.8±0.4 nM in normal cells (P < 0.005). A maximal stimulatory concentration of isoproterenol (100 nM) leads to the same increment in ion flux in erythrocytes from hypothyroid and normal turkeys. Analysis of the quantitative relationship between isoproterenol concentration, receptor occupancy, and associated effects upon potassium influx shows that at low levels of isoproterenol, where occupancy is linear with agonist concentration, occupation of a given number of beta receptors leads to a stimulation of potassium transport that is identical in erythrocytes from normal and hypothyroid turkeys. Thus, decreased sensitivity to catecholamine-stimulated potassium transport in hypothyroidism can be attributed to the decrease in receptor number and the resulting two- to threefold higher isoproterenol concentration required for occupancy of the same number of beta receptors. Once a single receptor is occupied, however, the more distal components of the sequence of events mediating the physiological response to beta-adrenergic agonists in the hypothyroid cell function as they do under normal circumstances. It would appear, therefore, that the decrease in sensitivity to isoproterenol-dependent ion flux in the hypothyroid turkey erythrocyte can be accounted for solely by the decrease in receptor number. These changes are shown to occur in the absence of any modifications in the number of Na⁺-K⁺ ATPase effector units per cell.