In the preceding paper (Berardi et al. Proc. R. Soc. Lond. B 251, 17 (1993)), it has been shown that nerve growth factor (NGF) prevents the functional and anatomical alterations induced by monocular deprivation (MD) at the level of the visual cortex. Here we report that an exogenous supply of NGF prevents the shrinkage of neurons in the deprived laminae of lateral geniculate nucleus (LGN). The soma size distribution for the deprived ipsilateral laminae of MD rats is shifted towards smaller sizes (mean percentage of shrinkage with respect to the ipsilateral undeprived lamina = 21%,s.d. = 2%). As in other mammals, MD affects LGN relay neurons and spares LGN neurons projecting to the monocular portion of primary visual cortex. In NGF-treated animals we found that the soma size distributions for the deprived and undeprived ipsilateral laminae extensively overlap. The results of the two papers show that an exogenous supply of NGF prevents MD effects at both levels, visual cortex and LGN, and suggest a role for NGF in the plasticity of the geniculo-cortical pathway.