Overfeeding Wistar rats starting one and a half months before sexual maturation led to a more marked adiposity than in the animals in which overfeeding was started after the occurrence of sexual maturity. The first category of the animals displayed an increase in the adipose tissue cell count, whereas rats overfed after reaching maturity showed an enlargement of these cells without increase in their number. Adiposity was more pronounced in rats given excessive food consisting mainly of carbohydrates in comparison with the animals of the same age chiefly given an excess of fats. The same rats showed an increase in the relative area of the islar tissue of the pancreas and of the beta-cell mass.