The functions of neurotrophins in relation to axon growth and branching during development of the nervous system are unknown. In order to address this question, we have investigated the influences of systemically administered mouse nerve growth factor (mNGF) and human recombinant neurotrophin-3 (hrNT-3) on dorsal root axon growth in the spinal cord of embryonic rats. As anticipated, mNGF has a marked influence on growth of dorsal root axons. In mNGF-treated animals, dorsal root axons in the developing dorsal funiculi and axon collaterals in developing gray matter are substantially longer than those of age-matched controls. Furthermore, growth cones of some dorsal root axons have more than twice the surface area of controls. These effects of NGF are highly selective. Dorsal root axons that occupy a lateral position in white matter and that normally give off collaterals to superficial dorsal horn are prominently affected. Axons that run medially in dorsal columns and that give off collaterals to laminae III and IV and the ventral horn are not demonstrably influenced by treatment with exogenous mNGF. In contrast to the striking effects of mNGF on dorsal root axon growth, the influences of hrNT-3 were considerably more complex. Administration of hrNT-3 increased the mean soma area of DRG neurons, particularly those at the larger end of the size spectrum, consistent with its hypothesized role as a growth factor for proprioceptive sensory neurons. However, in striking contrast to the actions of mNGF, hrNT-3 consistently inhibited axon collateral growth in gray matter at early developmental stages. At later stages, we could not discern a clear-cut influence of hrNT-3 on dorsal root axon growth and branching. We conclude that the ability of mNGF to stimulate axon growth in both white and gray matter is consistent with the idea that mNGF regulates the developing axonal projections of DRG neurons in vivo. In contrast, systemically administered hrNT-3 inhibits the axon collateralizations of DRG neurons in gray matter at early developmental stages. We hypothesize that this inhibitory effect may be related to disruption of a chemotropic gradient of NT-3, or to the widespread expression of the NT-3 receptor trkC, on non-neuronal cells.