A group of 16 mallard hens (Anas platyrhynchos), that had been given feed containing 40 ppm ofp,p'-DDE for 96 days, laid eggs with shells averaging about 15%?20% thinner than those of ten control birds during and up to 42 days after treatment. In eight of the treated birds killed at that time, whole-body DDE residues averaged 33.1 ppm (wet weight). The other eight treated birds and ten controls were kept through the winter with no additional DDE exposure and penned separately five days for individual egg collection about three weeks after laying began in spring. At that time (nearly 11 months after DDE feeding had stopped), the treated birds laid eggs with shells averaging 7.4% thinner than control eggshells (significant at P<0.05) and their whole-body DDE residues averaged 9.6 ppm (wet weight). Variations in eggshell thickness and DDE residues were considerable among treated birds. However, regression analysis showed moderate negative correlations (r=?0.51 to ?0.62) between eggshell thickness and DDE residues in whole bodies and eggs, and strong positive correlations (r=0.73 and 0.91) between DDE residues in whole bodies and in eggs.