Seven-Generation Study (P-F6): The concentration and total retention of dieldrin or p,p'-DDT and metabolites were determined in the total carcass of Swiss-Webster mice fed dietary supplements of aldrin 5 or 10 ppm, or DDT 100 ppm, to age 260 days. All groups showed a significant increase in total body retention (and concentration) of dieldrin or total DDT in the total carcass of the F1, F2, and F3 generations. Generally, these increases were related directly to increases in total body lipids, when compared with the P generations. The control (pesticide-free) diet was fed to all F4 generation experimental mice from weaning to age 260. The pesticides absorbed by these animals while in utero and via lactation were found, at the time of sacrifice, to have been excreted completely. When the experimental diets were resumed with the weanlings of the F2 generations, a repetition of the general findings in the P and F1 generations was noted-demonstrating that pesticide retention and total body lipids are closely interrelated, and that a high body lipid content favors a high retention rate of these fat-soluble pesticides. These results support our earlier studies in rats (Deichmann et al., 1972) and investigations with cirrhotic human livers with severe fatty infiltration (Oloffs et al., 1974). Conception became more delayed with each succeeding generation, requiring some degree of "selective" breeding of the F4, F5, and F6 generations.