Binding of the β-adrenergic ligands [3H]dihydroalprenolol and [125I]-cyanopindolol to pineal particulate fractions was increased 1- to 3.5-fold by addition of low concentrations of melatonin, α-adrenergic agonists, or α-adrenergic antagonists. Minimum concentrations of melatonin or α-adrenergic compounds which increased β-adrenergic binding were between 1 pM and 0.1 nM. The increased binding of [3H]dihydroalprenolol caused by melatonin (0.1 μM) was attributed to a major increase in Bmax, which persisted in protein fractions after removal of melatonin. Melatonin enhancement of [3H]dihydroalprenolol binding was apparent after 5 to 7 min (30°), was optimal between 20 and 40 min, and decreased at longer times. α-Adrenergic receptors are unchanged during β-receptor enhancement. © 1986 Academic Press, Inc.