Three groups of 6 adult male beagle dogs were fed aldrin 3 3 1,2,3,4,10,10-Hexachloro-1,4,4a,5,8,8a-hexahydro-exo,1,4-endo-5,8-dimethanonaphthalene; aldrin; HHDN; purity no less than 95%. Purchased from Shell Chemical Co., New York. and/or DDT 4 4 2,2-Bis(parachlorophenyl)-1 1 1,1-trichloroethane; chlorophenothane; DDT; recrystallized in our laboratory from technical DDT purchased from Matheson, Coleman, and Bell, Norwood (Cincinnati), Ohio. by capsule on 5 days of the week for 10 months, as follows. Group 1: 0.6 mg/kg aldrin; group 2: 24 mg/kg DDT; and group 3: 0.3 mg/kg aldrin plus 12 mg/kg DDT. Group 4 served as control. After this feeding period the dogs were observed for an additional period of 12 months. Hyperexcitability and tremors were noted in some of the dogs treated with 0.6 mg/kg aldrin, and with 0.3 mg/kg aldrin plus 12 mg/kg DDT. There were no significant serum cholinesterase or aliesterase changes. The concentrations of dieldrin 5 5 1,2,3,4,10,10-Hexachloro-6,7-epoxy-1,4,4a,5,6,7,8,8a-octahydro-1,4-endo-exo-5,8-dimethanonaphthalene; HEOD; dieldrin. in the fat, liver, and blood of the dogs were of similar magnitude for an individual tissue, whether the animals were treated with 0.6 mg/kg aldrin, or with the combined dosage of 0.3 mg/kg aldrin plus 12 mg/kg DDT. Retention of p,p′-DDT and DDE in fat and blood was roughly 2.5–4 times greater after administration of 12 mg/kg DDT plus 0.3 mg/kg aldrin than after administration of 24 mg/kg DDT without aldrin. The concentrations of p,p′-DDT and DDE in liver tissue were similar in both groups of animals.