Lens epithelial cells explanted together with their capsule into serum-free medium underwent cell division. The extent of cell division depended on the age of the donor rat. After explantation, lens epithelia from newborn and 5 day rats showed decreased mitotic activity from day 0 to day 3 whereas epithelia from 15 day rats showed a marked increase in mitotic activity which peaked at day 3. To determine the age when mitosis is first stimulated by explantation, explants were prepared from 10 day and 13 day rats. Explants from 10 day rats showed a slight increase in cell division and explants from 13 day rats showed a substantial increase in cell division. Therefore about 10 days of post-natal development, mitosis is stimulated by explantation and the magnitude of stimulation increased up to 15 days of age. We also found that in 7 week old rats the central epithelium, where most cells are blocked in Go-1 in vivo, showed a similar high level of cell division after 3 days. Moreover, the peripheral epithelium, which includes the zone of highest mitotic activity in vivo, had a significantly lower mitotic response to explantation than the central epithelium. Thus, explantation into serum-free medium must in some way stimulate cells blocked in Go-1 to re-enter the cell cycle. Furthermore, it is suggested that the increased mitotic response of rat lens epithelial cells with post-natal age indicates that the proportion of cells blocked in Go-1 increases substantially from 10-15 days of age.