Presence of substance P-like immunoreactive neurons and nerve fibers is demonstrated in the trigeminal ganglion of newborn and adult human subjects by the indirect immunofluorescence technique. Two populations of neurons containing high and low densities of immunoreactive material, respectively, are identified. Morphometric analyses indicate that (i) most of positive neurons are medium and small sized; (ii) immunoreactive perikarya grow in size from newborns to adults, with up to a 50% increase in diameter. Percent frequency of positive perikarya, on the other hand, is higher in newborns (23.6%) and decreases in adults (16.7%).