Endothelial cell counts made from specular micrographs of 1-month-old kitten and adult cat corneas demonstrate that a progressive increase in endothelial cell size and a reduced endothelial cell density occurs during the postnatal development of the cat cornea. In addition to confirming the difference in cell size, scanning electron micrographs show that kitten endothelial cells are much more pleomorphic than those of the adult. When the number of corneal endothelial cells/mm2 and the size of the whole cornea are calculated for the kitten and adult, hypertrophy rather than mitosis appears to be the principal mechanism responsible for maintaining a confluent endothelial cell monolayer during the postnatal development of the feline cornea. Hypertrophy also appears to play a role in establishing the adult corneal endothelial cell population of the rabbit when the previously published data of others are treated in a similar manner to those of the kitten and adult cat. Thus, endothelial cell hypertrophy plays a role in establishing an "adult" endothelial cell monolayer in species that have a widely divergent corneal endothelial cell mitotic capacity.