Atrophy and dysfunction of certain neurons, including cholinergic neurons in the basal forebrain, are key features of the neuropathology of Alzheimer's disease (AD). Since all individuals with Down syndrome (DS) develop AD neuropathology by the 4th decade, we reasoned that a genetic model of DS, the trisomy 16 (Ts 16) mouse, may provide an animal model to study the neurodegeneration in AD. Ts 16 mice fail to survive birth; to evaluate neurons for long periods in vivo required transplantation of fetal tissue. We previously demonstrated that Ts 16 basal forebrain cholinergic neurons (BFCNs) undergo age-related atrophy similar to DS and AD, and now show that a specific neurotrophic factor, nerve growth factor (NGF), acts to reverse Ts 16-induced atrophy of BFCNs and stimulates hypertrophy of these cells. As NGF levels were not decreased in the host, abnormalities intrinsic to Ts 16 BFCNs presumably caused the atrophy. Our results suggest that NGF may be useful in reversing cholinergic neurodegeneration in DS and AD.