The time of neuron origin in the trigeminal ganglion was examined in autoradiograms of 60-day-old rats that were exposed to a single pulse of 3H-thymidine on day 11, 12, 13, 14, or 15 of gestation. Heavily labeled neurons, representing cells in or near their last mitotic division at the time of the pulse label, were present in animals injected between embryonic days 11 and 13 with a peak on day 12. Within this time period, larger neurons were generated prior to smaller neurons with a peak for larger cells on day 12 and for smaller cells on day 13. Thus, the majority of trigeminal ganglion neurons are generated over a three-day period just after the midpoint of gestation. Neuron number, size, type, and cytoarchitectural organization were also examined in the ganglion. The mean neuron count per ganglion was 52,372. The size distribution of these cells ranged continuously from 7-61 microns (mean diameter) with no evidence for clearly defined subpopulations. The staining intensity and distribution patterns of the Nissl substance varied greatly from cell to cell precluding the classification of cells as light or dark. Little correspondence between these Nissl features and cell size was found. Among the clusters and rows of neurons in the ganglion, we did not see consistent cytoarchitectonic patterns which might reflect specific sensory receptive fields.