Nerve growth factor (NGF) has been proposed as a trophic molecule essential for the development of sympathetic and primary sensory neurones1,2. In newborn mice and rats, administration of nerve growth factor results in an increase in the number of surviving neurones3,4, whereas administration of antiserum to NGF decreases neuronal survival5,6. Thus it has been proposed that the factor is produced and secreted by the relevant target tissues to provide trophic support for the ingrowing nerves 1,7. The site of synthesis of nerve growth factor is still unknown, and it has been emphasized that a precise physiological role for the molecule cannot be ascribed until the cell types that produce it are known7-9. I report here the use of immunohistochemistry to localize endogenous NGF in the rat iris, a tissue in which there is sound biochemical evidence for the production of NGF activity10. Surprisingly, the results reveal that NGF can be detected readily in Schwann cells, but not in smooth muscle cells of the iris when it is sympathetically denervated or cultured.