Fenoxaprop-P-ethyl (AE F046360) is one of the two optically active enantiomeric forms of Fenoxaprop-ethyl (AE F033171), the R(+)-form, which is identical with the D(+)-configuration at the chiral centre. As stated in the comprehensive summary of Tarara, G., 1999b (Fenoxaprop-P-ethyl: Metabolic fate of the 4-hydroxyphenoxypropionic acid moiety and discussion of the radiolabel position; Report OE98/124, Document C005529) the basic behaviour in non-chiral environments (e.g. water solubility, partition coefficient in octanol/water, aqueous hydrolysis) is the same for both enantionmers. Therefore tests on the racemate are also useful to describe the behaviour of one of its enantiomers. Fenoxaprop-P-ethyl belongs to the structural class of heteroaryloxy-phenoxypropionic acids and thus is a well degradable substance. Degradation is shown to proceed basically via two major steps: A rapid ester hydrolysis under formation of the active metabolite Fenoxaprop-P (AE F088406) is followed by a cleavage at the central heterocyclic ether bond. The result is a split of the molecule into two main parts, the chlorobenzoxazolone (AE F054014) and hydroxyphenoxypropionic acid (HOPP-acid, AE F096918). A minor pathway is the degradation of the Fenoxaprop-P to the phenolic metabolite (AE F040356), e.g. the possibility of a cleavage at the alkyl-phenyl ether bond. For HOPP-acid this metabolic step would result in the formation of hydroquinone and hydroxypropionic acid. The hydrolysis at the central ether bond, e.g. loss of the heterocycle, results in a complete loss of biological activity. This ether bond cleavage can be induced either by biotic or abiotic hydrolysis. Two significantly different radiolabel positions (U-14C-chlorophenyl- and 2-14C-dioxyphenyllabel) are possible and both were used to investigate the metabolic fate of these major degradates. Halogene-containing heterocyclic systems like the benzoxazole moiety in Fenoxaprop are rarely known in natural biological processes (Tarara, G., 1999b: Fenoxaprop-P-ethyl: Metabolic fate of the 4-hydroxyphenoxypropionic acid moiety and discussion of the radiolabel position; Report OE98/124, Document C005529). This moiety was therefore regarded as the most critical part for exposure assessment. In soil metabolism studies it was shown that this major metabolite was preferably bound to organic matter. From this organic fraction it could be released only by strong acids. Non-extractable residues accounted for up to 70% AR after 100 days in studies with the U-14C-chlorophenyl-label, 14CO2 amounted for up to 32% AR. In a study with 2-14C-dioxyphenyl-label the HOPP-acid (AE F053022, study conducted with racemic mixture) was only found as minor metabolite, indicating a transient character of this metabolite. A lower formation of bound residues with this radiolabel was accompanied by a higher mineralization rate. 14CO2 amounted for up to 55% AR after 64 days. Non-extractable residues amounted for up to 32% AR. Residues originating form the dioxiphenyl moiety of the parent compound are obviously significantly faster mineralised to CO2 than those originating form the application of the chlorophenyl-label.