Short-term receptor regulation by agonists is a well-known phenomenon for a number of receptors but little is known about the regulation of α1-adrenergic receptors. In the present study we provide evidence of α1-adrenergic receptor changes induced by agonists on DDT1 MF-2 smooth muscle cells. The cells were preincubated with the agonist and receptor changes were investigated in the cells washed free of the agonist. On cells pretreated with norepinephrine the number of receptors recognized by [3H]prazosin at 4°C was reduced by 38%. The receptors were not degraded as the number of sites was the same in control and norepinephrine-treated cells when binding was measured at 37°C. When binding was measured on fragmented membranes (at 4°C), the number of receptors was the same in control and norepinephrine-treated cells, suggesting that the disruption of cellular integrity might expose receptors which are probably sequestered after agonist treatment. We conclude that agonists induced rapid sequestration of receptors on intact DDT cells.