Aldrin and dieldrin are the common names of insecticides containing 95% HHDN and 85% HEOD, respectively. Throughout this monograph the names aldrin and dieldrin are used, although concentrations determined in the different matrices are actually those of the active molecules HHDN and HEOD. Aldrin is readily metabolized to dieldrin (HEOD) in plants and animals. Only rarely are aldrin residues present in food or in the great majority of animals, and then only in very small amounts. Therefore, national and international regulatory bodies have considered these two closely related insecticides together. The practicality of considering them jointly is further emphasized by the lack of significant difference in their acute and chronic toxicity and by their common mode of action.