Metabolism of the chrysanthemates (S)-bioallethrin, cinerin I, jasmolin I, and pyrethrin I by NADPHdependent oxidases of mouse liver microsomes yields 13-18 metabolites in each case oxidized a t the methyl, methylene, and alkenyl substituents to form alcohols, aldehydes, carboxylic acids, epoxides, and dihydrodiols. Rat microsomes are more specific than mouse microsomes in hydroxylating the @)-methyl substituent of the 2-methylpropenyl moiety compared with other molecular sites. Metabolites in the urine of allethrin-treated rats include compounds modified in both the 2-methylpropenyl and allyl moieties as free carboxylic acids and glucuronides. The pyrethrates cinerin 11, jasmolin 11, and pyrethrin I1 undergo microsomal hydrolysis of the methoxycarbonyl group and oxidation of the butenyl, pentenyl, and pentadienyl substituents to alcohols, epoxides, and dihydrodiols. Metabolites of these chrysanthemates and pyrethrates are tentatively identified by chemical ionization mass spectrometry following treatment with diazomethane or diazoethane and bis(trimethylsily1)acetamide and separation by high-resolution gas chromatography with hydrogen as the carrier gas.The six natural pyrethrins in pyrethrum extract and their synthetic analogue (SI-bioallethrin (i.e., the rethrins) are important insecticides for control of household and stored products pests (Casida, 1973). The metabolic fates (S)-bioallethrin (A): R, = CH,, R, = H pyrethrin chrysanthemate (I) pyrethrate (11) cinerins (C) CI: R, = CH,, CII: R, = CO,CH,, --R, CH,-R, = CH, R, = CH,CH, R, = CH=CH, jasmolins (J) JI: R, = CH,, JII: R, = CO,CH,, pyrethrins (P) PI: R, = CH,, PII: R, = CO,CH,, R, = CH,CH, R, = CH=CH, of A and PI, partially defined in microsomal oxidase systems and in rats, involve multiple sites of oxidation with little or no cyclopropanecarboxylate hydrolysis (Casida et al., 1971;Elliott et al., 1972). Comparable information is not available on the other pyrethrum constituents except for their relative ease of oxidation in microsomal oxidase systems (Soderlund and Casida, 1977a). With molecules as complex as the rethrins it is difficult to predict the effects of relatively small structural modifications (e.g., R, = CH,, CO,CH,; R2 = H, CH,, CH,CH,, CH=CH,) on their rates and sites of biodegradation. In further evaluating the comparative fate of the rethrins, it is important to use analytical methods applicable to all of the relevant compounds. The pyrethrum constitPresent address:uents and A are conveniently analyzed by high-resolution gas chromatography (HRGC) with an electron-capture detector (ECD) or HRGC-chemical ionization (CI) mass spectrometry (MS). These methods are applied here to the rethrin metabolites, after appropriate derivatizations. This study compares the metabolic fate of A and the six natural pyrethrins in mouse and rat liver microsomal oxidase systems and of A in rats.