(1) This account presents the results of a series of studies on variations in the mitotic index of frog lens epithelium.(2) Seasonal variations in mitotic index have been found in the lenses of R.catesbeiana and R.pipiens. Though the numbers of division figures vary considerably, it can be concluded that in the winter months cellular reproduction is a comparatively rare event in the epithelium of both species. By contrast, in the spring, the number of mitoses is much higher. (3) It is suggested that temperature may be an important factor in regulating the MI; possibly more important than endogenous factors implicated by many workers for other types of circadian and circannual variation (4) Animals maintained at 24 °C during the coldest part of the year manifest a normal range of proliferative activity while in frogs maintained in the cold during the warmer periods mitosis is abolished. (5) Additionally, animals which have arrived at the laboratory after having been exposed to cold temperature (in transit), have mitotic indices which are increased as some function of the number of days acclimation to room temperature. (6) What is more, during the calendar time of hibernation, the proliferative response elicited by injury or in vitro culture seems to be enhanced if the animals from which the lenses are obtained are acclimated for long periods. (7) No diurnal variation was discerned in the lenses of these animals. This finding disagrees with the results from similar studies on mammalian lenses. (8) In agreement with studies on mammalian lenticular material was our finding that there is some relationship between age and MI. However, this becomes obvious only when one compares very young (i.e. 2.5–11 months post-metamorphosis) to adult organisms (24 + months post-metamorphosis). (9) These results were evaluated and discussed in relation to previous work from our laboratory as well as to relevant studies from other groups. A number of seemingly appropriate follow-up investigations were recommended.