The endothelium of the cornea is said to retain its cellular proliferative capacity in rabbits but not in cats or primates. The present study was undertaken to determine whether the decline in cell density with increasing age is shared by all species that presumably lose their corneal endothelial-cell proliferative capacity and to determine whether in primates age-dependent changes in the endothelium are influenced by such environmental factors as light intensity and temperature. We found that in cats a very rapid decline in endothelial cell density during the first 10 months post partum was followed by a slower decline between 10 and 12 months, corresponding to a rapid increase followed by a slower rate of increase in corneal surface area. Between 1- and 14 years of age, a further decline in endothelial-cell density did not occur, and no endothelial pleomorphism or corneal guttata was observed among the 44 cats studied in this age group. In rhesus monkeys, an approximate 20% decline in cell density between 1 and 6 years of age, representing a period of ocular growth, was followed by a continuing decline in endothelial cell density at a slower rate of 0.7 to 0.8% per year throughout the rest of the lifespan, despite the fact that there was no evidence for further increase in globe size during this period of adulthood and aging. This rate of decline is similar to that described in the human corneal endothelium and was found to be the same in free-ranging animals continually exposed to high levels of solar radiation and in animals caged under artificial room light of much lower intensity and a year-round temperature of 21 +/- 1 degrees C. However, the animals maintained indoors showed a much higher frequency of endothelial pleomorphism than the free-ranging animals. These differences in age-dependent changes between feline and primate corneal endothelium and between primates living under grossly different environmental conditions are discussed.