The influences of neurotrophic factors on adult mammalian spinal cords are incompletely understood. In the present experiment, we utilized somatic gene transfer to examine the effects of nerve growth factor (NGF) on the unlesioned spinal cords of adult Fischer rats. Fischer 344 rat primary fibroblasts were genetically modified in vitro to produce and secrete NGF, then grafted to spinal cords at the T7 level. Grafts survived in vivo for periods of up to 1 year, and induced an extremely robust ingrowth of spinal neurites. Control and basic fibroblast growth factor-producing grafts did not promote extensive neurite growth. Neurites penetrating the NGF grafts were of sensory origin, since they labeled immunocytochemically for calcitonin gene-related peptide but not markers of other neuronal transmitter phenotypes. Electron microscopy revealed that neurites within NGF-secreting grafts were enveloped in glial cell processes and that axons frequently became myelinated. These results indicate that (i) genetically modified cell grafts are a useful model for studying trophic factor effects in the adult mammalian spinal cord, (ii) sensory neurites maintain robust NGF responsiveness into adulthood, and (iii) sprouting neurites can follow glial channels and become myelinated in the adult spinal cord. Grafts of fibroblasts genetically modified to secrete trophic factors merit study as potential tools for promoting regeneration after spinal cord injury.