The use of immunofluorescence with affinity-purified antibodies enabled cytological localization of nerve growth factor-like [NGF] material in the rat. Immunoreactivity was observed along various nerve tracts of the fetal rat brain and spinal cord at day 15 of gestation. Longitudinal pathways in ventral and dorsal spinal cord, ventral lower brain stem, posterior commissure, retroflex fascicle and in the olfactory bulb were all positive. A weaker and more widely spread immunostaining was visible in many areas in the CNS. Cranial nerves were strongly immunoreactive. Neuronal perikarya in the retina and the olfactory mucosa as well as filae olfactoriae and olfactory nerve all the way to the olfactory bulb were also positive. In sensory ganglia and peripheral nerves most immunoreactivity was confined to supporting tissues, probably including Schwann cells. In irides, the pattern of immunoreactivity was similar to that of the sensory and autonomic innervation. More intensively fluorescent material was found in regrowing nerve fibers in iris transplants. NGF and/or a related protein is present in large amounts along nerve pathways in supportive tissues of the peripheral nervous system as well as in the CNS during early development.