Nerve growth factor corrects developmental impairments of basal forebrain cholinergic neurons in the trisomy 16 mouse. The trisomy 16 (Ts16) mouse, which shares genetic and phenotypic homologies with Down syndrome, exhibits impaired development of the basal forebrain cholinergic system. Basal forebrains obtained from Ts16 and euploid littermate fetuses at 15 days of gestation were dissociated and cultured in completely defined medium, with cholinergic neurons identified by choline acetyltransferase (ChAT) immunoreactivity. The Ts16 cultures exhibited fewer ChAT-immunoreactive neurons, which were smaller and emitted shorter, smoother, and more simplified neurites than those from euploid littermates. Whereas the addition of beta-nerve growth factor (100 ng/ml) augmented the specific activity of ChAT and neuritic extension for both Ts16 and euploid cholinergic neurons, only Ts16 cultures exhibited an increase in the number and size of ChAT-immunoreactive neurons. Furthermore, Ts16 ChAT-immunoreactive neurites formed varicosities only in the presence of beta-nerve growth factor. 