1. The secretory activity of the Harderian gland in Rana esculenta varies during the year, reaching its highest activity during the hottest period (July-August). Therefore, secretion may be modulated by temperature and/or photoperiod. 2. Adult males and females were placed under several combinations of light and temperature in two different periods of the year (February and July) in order to elucidate their respective roles, if any, on the stimulation of secretion. 3. Under experimental conditions, high temperature (24 degrees C), irrespective of the photoperiod selected, stimulates secretion shown both at histological and histochemical levels. 4. Low temperature (8 degrees C) impairs secretory activity, again independently of the photoperiod selected. 5. This data suggests that the secretion of the Harderian gland in Rana esculenta is modulated mainly by temperature.