Epididymal fat pad adipocyte frequency distributions of obese (fa/fa) and nonobese (non-fa/fa) Zucker rats were determined from 2 to 20 wk of age. The fa/fa rats consistently displayed a higher proportion of larger adipocytes than the non-fa/fa rats of all ages studied. Increased adipocyte size preceded an increase in the number of adipocytes during the early phases of obese development. Among the fa/fa rats, the frequency of small cells continued to increase, which resulted in a bimodal distribution by 20 wk of age. Discriminant analysis suggests that the differences in adipocyte size frequency distributions at 2 wk of age may be another useful means to identify fa/fa and non-fa/fa pups prior to expression of overt obesity. Increased fat storage may reflect the lower energy expended by the very young fa/fa rats.