Two populations of neurons are present in the dorsal root ganglia: i) the first which is connected with the neuromuscular spindles is localized in the mediodorsal (MD) part of the ganglion; ii) the second which is connected with the cutaneous receptors is localized in the ventrolateral (VL) part of the ganglion. The growth of MD but not of VL neurons in vitro is dependent upon the presence of the NGF. In order to study the presence in vivo of such differential effect of the NGF on the two neuronal populations, we have injected rat embryos with NGF antiserum (AS-NGF) and recorded the compound action potentials of different hindlimb nerves. We found that the sensory component, in the studied muscular nerves of the hindlimb, is seriously depressed in treated animals. These results indicate that the animals treated with AS-NGF suffer of a depression of activity in the afferents from the muscle spindles. This is probably due to a decrease in the number of these fibers in their muscle nerves.