Aldrin applied to tissue pieces of field-grown radishes, beets, turnips, rutabaga, carrots, and parsnips was converted to dieldrin. The epoxidation rates were different for different crops, but varied little for the same plant species grown in two consecutive years and treated and incubated in the laboratory under identical conditions. The reaction rates remained constant with time between 24 and 72 hours of incubation. Destruction of the cell structure by grinding or freezing prevented the formation of dieldrin. Heat, very low partial pressure of O2 during incubation, and addition of KCN and 2,4-DNP also inhibited the reaction. Aldrin applied to the surface of roots or stems of young pea plants (5 to 7 days of germination) resulted in the production of dieldrin. Reaction rates were significantly higher with roots than with stems; most activity was noticed with root tips and least with the upper parts of pea root. © 1969, American Chemical Society. All rights reserved.