When frogs were exposed to cold temperature (4°C) the number of mitotic figures in the lens epithelium was reduced by 80–100%. However, mitotic activity does not vary directly with temperature. It is likely that other factors, such as hormones, also regulate proliferation in the germinative zone. Light-dark influences and diurnal changes have, on the other hand, not been observed to produce obvious effects. Implants of pituitary glands or lyophilized pituitaries stimulate cell division as do LH, TSH, T3 and T4. No steroids have yet been found effective in this regard. Somatotrophin and adrenaline are likewise ineffective. The stimulation by LH is not the result of a nonspecific immunological response, for bovine serum albumin injections exerted no detectable influence upon proliferation in the lens. When animals were transferred from 4 to 24 °C the MI increased substantially. This increase occurred on the first day of the transfer in summer frogs but took 8 days in winter frogs. Radioimmunoassay demonstrated a correlation between the number of mitosis and LH. In the transfer studies no correlation was found between serum concentrations of FSH, T3 or T4 and mitotic activity. A thermoendocrine hypothesis was presented to help explain the results obtained to date as well as to aid in the formulation of suitable experiments.