The time course and specificity of neuron addition to lumbar dorsal root ganglia (DRGs) L4–L6 of rats was investigated. By using methods validated by three-dimensional reconstructions, profile counts in paraffin sections of nucleoli within a nucleus were 36% greater in 100-day-old (P100) rats than in 1-day-old (P1) rats. Adult values were reached by P50. Added neurons fell disproportionately into the population of neurons whose size was below that of the mean size within the ganglion. The biochemical characteristics of small neurons were used to determine whether added neurons fall into particular subpopulations. In DRGs, L3 and L4, the number of neurons immunoreactive to substance P (SP) or calcitonin-gene-related peptide (CGRP) or that bound the lectin isolectin B4 (IB4) was determined. Between P5 and P100, the number of SP-stained neurons increased by 2,280 (40% increase), CGRP-stained neurons increased by 6,080 (70% increase), and IB4-stained neurons increased by 6,900 (90% increase). The increase in the number of neurons stained for CGRP or IB4 was more than twice the number of neurons found to be added to these ganglia, indicating that coexpression of these markers as well as neuron number may be developmentally regulated during postnatal life. J. Comp. Neurol. 449:158–165, 2002. © 2002 Wiley-Liss, Inc.