Regain of body weight in European Hamsters after fasting was investigated at various periods of the year and under different ambient conditions (fig. 1) with and without radiothyroidectomy. 1. Weight gain after food deprivation proceeds at rates which vary with the season. It is faster in spring and summer than in winter. 2. Variations in ambient temperature and in light conditions do not influence the daily weight gain and the relative rate of weight increase (fig. 2,3). 3. Radiothroidectomy depresses these two parameters in winter as well as in spring, and lowers the level of the set point of the regulating mechanism (fig. 4,5).