We investigated the neuroprotective effects of 17β-estradiol (E2) on medial septal cholinergic neurons following partial unilateral lesion of the fimbria-fornix. Adult female rats were ovariectomized (OVX) and, 5 days later, treated with a single intravenous (iv) injection of an estradiol (E2)-chemical delivery system (E2-CDS) or its vehicle hydroxypropyl-β-cyclodextrin (HPCD). All rats were subjected to partial unilateral electrolytic fimbrial lesion the following day. At 20 days postlesion, brain slices from treated animals were assessed for choline acetyltransferase (ChAT) by immunohistochemistry. Animals treated with HPCD or E2-CDS showed a 44 or 4% decrease, respectively, in ChAT-positive neurons on the lesioned side compared to the nonlesioned side of the medial septum. In a second study using the same lesioning procedure, adult OVX rats received either a subcutaneous E2pellet implant (n=6), or, 5 days postovariectomy, a single iv injection of E2-CDS (n=8) or HPCD (n=6). Animals treated with HPCD showed a 55% decrease in ChAT-positive neurons on the lesioned side compared to the nonlesioned side of the medial septum. By contrast, rats treated with E2-CDS or E2pellet had a 14 or 13% decrease, respectively, in ChAT-positive neurons. Interestingly, E2treatment substantially decreased ChAT-positive neurons on the nonlesioned side of the medial septum in comparison to control animals. The present study suggests that cholinergic neurons in the medial septum are protected from lesion-induced degeneration by treatments which increase brain E2levels. Thus, E2may play a neuroprotective role in the basal forebrain cholinergic system.