Denervation of the hippocampal formation in adult rats through lesion of the medial septum and diagonal band or by transection of the fimbria/fornix elicits an increase in the number of putative alpha-adrenergic receptor binding sites labeled by the antagonist ligand [3H]WB4101 [2-(2,6-dimethoxyphenoxyethyl)-aminomethyl-1,4-benzodioxane]. This increase in [3H]WB4101 binding is observable at 6 days postlesion, preceding the ingrowth of sympathetic axons into the partially denervated regions of the hippocampus. The receptor up-regulation is specific for lesions of the septal (primarily cholinergic) innervation of the hippocampus. Damage to noradrenergic, dopaminergic, or serotonergic afferents as well as kainate injections in the lateral septum had no effect on [3H]WB4101 binding levels. In vivo muscarinic/cholinergic-receptor blockade does not mimic the effects of the lesion on receptor binding levels or upon axonal sprouting of the sympathetic neurons. Although [3H]WB4101 binding consistently increased after septal deafferentation, there was no clear-cut effect upon the other adrenergic ligands, including [3H]prazosin, [3H]yohimbine, P-[3H]aminoclonidine, or [3H]dihydroalprenolol. These observations can be interpreted as demonstrating a unique and selective adrenergic receptor increase after a nonadrenergic denervation but accompanying the ingrowth of anomalous adrenergic fibers. We suggest several possible relationships between the new binding sites and the ingrowing axons.