The influence of NGF on cholinergic interneurons in organotypic roller tube cultures of 4 day postnatal rat striatum was examined after 13 to 16 days in vitro. Cultures were divided into four groups. The medium of the NGF treated group was supplemented with 5 ng/ml NGF, whereas control groups were cultured either without NGF, by adding 20 ng/ml neutralising anti-NGF antibody, or by adding both NGF and anti-NGF antibody to the medium. Two different cell populations were identified by an image analysis system which measured acetylcholinesterase staining intensity. It was demonstrated that NGF promotes survival of the large, intensely stained population. Eighty computer-assisted reconstructions of intensely stained cells, 20 for each treatment group, were performed in a random order by means of a neuron tracing system. Axons and dendrites were analysed separately. NGF enhanced complexity of neuritic, predominantly axonal trees by increasing the number of axonal segments by 91% to 100% (P < 0.01), the number of dendritic segments by 33% to 63% (P = 0.09 to P < 0.01), maximal axonal branch order by 37% to 50% (P < 0.05), and maximal dendritic branch order by 22% to 37% (P < 0.05). Further evidence of more complex neuritic trees was given by Sholl concentric sphere analysis. Anti-NGF antibody could block all these effects. General rules of branching architecture were not affected by NGF treatment as shown by analysing mean segment length in relation to the branch order, branch point exit angles, total tortuosity, Rall's ratio, and tapering of neuritic trees.