Double immunofluorescence studies using antibodies against NF-L and peripherin revealed three distinct subpopulations of neurons in rat dorsal root ganglia (DRG). In the adult rat, 46% of the DRG neurons were small and peripherin-positive (NF-L-negative), and 48% were large and NF-L-positive (peripherin-negative). About 6% were both peripherin- and NF-L-positive. All of the DRG neurons reacted with antibodies to NF-M and nonphosphorylation-dependent or phosphorylation-independent antibodies to NF-H. The neuropeptides were predominantly found in the peripherin-positive small cell population. Eighty-seven percent of the peripherin-positive small cell population contained substance P immunoreactivity, while 43% of this cell population contained CGRP. In contrast, only 18-24% of the NF-L-positive large-cell population contained neuropeptides, and these were primarily in a smaller sized subpopulation. Similar patterns of antigen representation were observed in neonatal (PN2) DRG cell populations. Tissue cultures of sensory ganglion cells from PN2 DRG, in serum-free medium, stably maintained exclusively peripherin-positive neurons, with about 5% of these containing coexistent NF-L immunoreactivity. Very high levels of neuropeptide gene expression were exhibited by these postnatal neurons in culture.