To gain some insight into possible functions of nerve growth factor (NGF), we suppressed the endogenous levels of NGF in newborn rats by subcutaneous injections (3 μl/g body weight) of rabbit antibodies to purified mouse β-NGF (ANTI-NGF). Fiber and axonal areas and perimeters were measured for unmyelinated and myelinated sensory fibers in T9 dorsal roots (DR) in three groups of animals: (1) ANTI-NGF treated littermates, (2) preimmune sera treated littermates (PREIMM), and (3) untreated littermates (UNTR). In some rats, fibers in ventral roots (VR) were measured and, in other rats, sensory processes in peripheral nerves (PN) were measured following radical ventral rhizotomy. The only outer area and perimeter measurements that were statistically different were those in the ventral root (P < 0.013 and P < 0.043, respectively). However, myelin thickness was significantly thinner in the dorsal roots of the ANTI-NGF group than in the dorsal roots of the UNTR and PREIMM groups (P < 0.000009 and P < 10−6, respectively). Myelin thickness in the ventral roots of the ANTI-NGF group was also statistically thinner than that in the UNTR group (P < 0.001). There were no statistically significant differences when comparing the UNTR group to the PREIMM group. In the peripheral nerves studied, there was no significant change in the myelin thicknesses between the ANTI-NGF and UNTR groups of animals. These results indicate that Schwann cell-neuronal interactions are altered by the inactivation of NGF, and that (1) the central processes of sensory fibers are affected and not the peripheral processes and (2) motor fiber myelination is altered. Therefore, we propose that NGF is not only a neurotrophic and neurotropic molecule but is also an important instructive molecule for the initiation and progress of normal myelination.