Cell size and number of parametrial fat pads were determined in Swiss mice made obese by means of a high-fat diet (40% lard w/w) given ad lib. This diet and a control were introduced to two groups of mothers during gestation and lactation, and sucklings were given the same diets as their mothers at weaning and throughout life.2-wk old mice suckled by mothers fed a high-fat diet have fatter parametrial pads. This difference is due solely to an increase in fat cell size. After weaning, until the 18th wk, the two groups differed with a striking fat cell enlargement seen in the obese group. Later on, whereas cell numbers did not change in the control group, a constant and uninterrupted increase in number is shown in those of obese mice until the 52nd wk. Hyperplasia was observed only in adults. When the high-fat diet was introduced to adult rats it also triggered an increase in fat cell number. Three sites of fat pads were compared in both sexes at 32 wk of age. All sites increased in weight in the high-fat fed group. This was due to: hyperplasia in perirenal site, hypertrophy in epididymal and subcutaneous sites, and hyperplasia plus hypertrophy in the parametrial one. So, in each sex, adipose sites in the obese mice reacted to the diet in a site-specific way. It was concluded that the level of fat in a diet is involved in both formation and maturation of new fat cells and in the regulation of fat cell lipid content. The two processes may be separated or may act together according to the adipose tissue site.