Soil metabolism studies were performed with chloridazon and with its two metabolites, B and B-1. Under aerobic conditions in soil chloridazon is primarily degraded to its main metabolite B which contains the pyridazinone ring part of the molecule. This metabolite can account for more than 50 % TAR at the end of the tests evaluated and is considered stable. Whereas the phenyl ring part of chloridazon is microbially attacked, opened and mineralised to CO2 (up to 76 % TAR after 30 days), the pyridazinone ring persists undergoing only negligible ultimate degradation of max. approx. 5 % TAR after 120 days. Bound residues reach a max. of 19 % TAR after 367 days. In aerobic soil metabolism studies conducted with metabolite B, unchanged metabolite B accounted for up to 51 % TAR after 121 days. Apart from insignificant metabolism/ degradation the further fate of metabolite B consists predominantly in the formation of high amounts of bound residues, precisely up to 42 % TAR after 92 days, whereas only small to moderate amounts of metabolite B (max. 14 % TAR after 121 days) are converted into metabolite B-1 by methylation of the pyridazinone nitrogen N-2. This process is obviously reversible as it was shown in a separate metabolism study with metabolite B-1 that a partial conversion from metabolite B-1 back to metabolite B (max. 10.5 % TAR after 120 days ) is possible. The reaction sequence between methylation and demethylation is a well known physiological activity of soil microorganisms. As already described for the metabolite B, beside up to 59 % TAR unchanged metabolite B-1 identified after 120 days, the major portion of metabolite B-1 was also recovered by combustion as non-extractable radioactivity, precisely up to 37 % TAR after 120 days. Ultimate aerobic degradation of both metabolite B and metabolite B-1 is negligible with max. CO2-amounts of approx. 5 % TAR released from each molecule after 120 resp. 121 days of testing. In conclusion, from the results of the metabolism studies performed, the behaviour of chloridazon in soil consists in the formation of one major metabolite, which under aerobic conditions is to a small extent further metabolised, and the extensive formation of bound residues, whereas mineralisation and hence ultimate biodegradation to CO2 is insignificant. Thus chloridazon and its metabolites are considered highly persistent in soil.