Feeding tests were conducted from January, 1966, to January, 1969, to ascertain the effects of daily ingestions of sublethal amounts of dieldrin on white-tailed deer (Odocoileus virginianus). Groups of deer on 0 ppm dieldrin (controls), 5 ppm, and 25 ppm dieldrin were maintained at these respective levels, as were their progeny. Treated food was readily accepted. Dieldrin intoxication was not observed, and 9 of 10 animals of each group survived 3 years of treatment. No differences in conception or in utero mortality were found between groups. Fawns from dieldrin-fed does were smaller at birth and greater post-partum mortality occurred. Fertility of male progeny was not affected. Growth was slower and remained reduced in dieldrin-treated females which were immature when the study began. Hematologic values and serum protein concentrations were not significantly (P > 0.05) related to treatment. Liver/body weight ratios were significantly (P < 0.05) larger for the 25-ppm-dieldrin group. Pituitary glands were smaller and thyroids were larger in dieldrin-fed deer. Weight gains of fawns were significantly (P < 0.05) reduced 2 of 3 years in dieldrin-treated groups. Placental transfer of dieldrin occurred. Whole milk from does fed 25 ppm dieldrin contained residues of 17 ppm. Residue levels in brain, liver, and thigh muscle tissues showed no evidence of increasing with length of treatment, but showed definite relationships to levels of dieldrin in daily diets. Nursing fawns had higher residues in brain tissues than did older deer on 5 ppm a d 25 ppm dieldrin. Highest brain residues (12.60 and 12.10 ppm, wet weight) occurred in fawns only a few days of age at death. Equilibrium between ingestion and storage or excretion of dieldrin occurred prior to 200 days and continued until nearly 1,100 days. There was no evidence of a sharp decline in residues after a long period of continued dosage. Daily ingestion of 100 and 200 ppm of dieldrin proved fatal to yearling male deer at 27 and 22 days, respectively.