HAPAB The action of nonliving environmental systems, light, wind, moisture and other environmental factors on the metabolism of pesticides is discussed. Light, either sun or ultraviolet, is the most important fac tor. For example, after exposure to sunlight, grass treated with dieldrin was found to contain an isomer in which the methylene bridge had become bonded to an adjacent carbon atom. Aldrin undergoes similar self-condensation and phosphorus insecticides such as mevinphos undergo various types of photoisomerization. Sevin is comverted to a number of decompositionproducts, including 1-naphthol. Parathion as well as several similar organophosphorus insecticides are transformed into more polar metabolites and sulfur-containing side chains are prome targets for photooxidation. Air converts heptachlor and aldrin to the corrsponding epoxides on plant surfaces, pyrethrin forms a peroxide with subsequent polymerization of the diene side chain and rotenone likewise reacts rapidly with air to form dehydrorotenone. Phosphorothionates are converted to the oxygen analogs, phosphites from phosphates, sulfoxides from sulfones and dithiocarbamates give thiuram disulfides. Water hydrolyzes esters, amides and carbamatesto form acids, alcohols or nitrogenous products. Organophosphorus esters under acidic or neutral conditions, experience attack on the alkyl-oxygen bond, whereas alkaline conditions result in breaking of the phosphorus- oxygen bond. DDT-type compounds readily lose HCl to form DDE-type derivatives. Soil pH and organic matter also play a part in the metabolism of pesticides. Heat will cause DDT-type pesticides to lose HCl. N-Methylcarbamate insecticides are quantitatively decomposed to phenol and low-boiling methyl isocyanate; transalkylation may occur under very mild conditions. Many nucleophilic groups present in proteins and nucleic acids in water at a pH of 7.5 have highly reactive sites that may be alkylated by many types of pesticides. Environmental factors often act in conjunction with each other. The interaction of the effect of light, air and water is illustrated by 2,4-D. Aqueous solutions of this herbicide are photolyzed, through a series of reactive intermediates, with replacement of the chlorines by hydroxyl groups and cleavage of the ether bond. Then, in a nonphotochemical oxidation, 1,2,4-benzenetriol is oxidized to a nontoxic polymeric humic acid. On damp filter paper in sunlight, this conversion requires only a few minutes. TOXICOLOGY AND PHARMACOLOGY 69/12/00, 469 1969