A method is described for the analysis of cell types in mouse dorsal root ganglia using the distribution of cell cross-sectional areas measured at the level of the nucleolus in 1.5 μm Epon sections. Using a computer program it was possible to demonstrate the existence of two normally distributed sub-populations of neurons in all the 3rd lumbar segment ganglia (17 in number) measured at various ages from birth to 70 days. The two populations appeared to correspond with large light cells and small dark cells. The large light cell bodies increased in size until about 20 days postnatal, subsequently their size decreased whereas the mean size of the small dark cells reached a plateau by about day 10. The relationship of both nuclear volume and surface area to the surface area of the perikaryon differed between light and dark cells. The number of neurons in L₃ remained virtually constant at about 6000 throughout the period examined. Since the proportion of neurons in each population was not shown to change with age there was no evidence that cells could change from one type into the other.