Abstract The administration of an i.p. dose of phenylephrine (2 mg/kg) increased the number of [ 125I]Tyr 11-somatostatin ([ 125I]Tyr 11-SS) receptors and decreased their apparent affinity in rat hippocampal membranes 7 h after its injection. Prazosin (20 mg/kg, i.p.) administered 1 h before phenylephrine reversed effects of the latter on SS binding. Prazosin alone decreased the number of SS receptors without changing the affinity. The addition of phenylephrine or prazosin (10 −5 M) to the incubation medium did not change the SS binding characteristics. The present results support the notion that the α 1-adrenergic system regulates the binding of SS to its specific receptors in rat hippocampus.