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Biotransformation may be responsible for the lack of bioaccumulation of a number of polychlorinated dibenzo‐p‐dioxins (PCDDs) and dibenzofurans (PCDFs). Goldfish (Carassius auratus) that were exposed to PCDDs and PCDFs, and piperonylbutoxide (PBO) in water, bio‐concentrated significantly more congeners than goldfish exposed to PCDDs and PCDFs only. Mono‐oxygenase activity, which is responsible for the oxidation of specific PCDD and PCDF congeners in untreated fish, was inhibited by fish treated with PBO. In the PBO‐treated group and in the control group, congeners with all lateral positions substituted were found. Congeners that lack chlorine substitution on one or more of the lateral (2,3,7,8) positions and congeners that have all lateral positions chlorinated were found only in PBO‐treated fish. Congeners that have at least one free lateral position were therefore assumed to be biotransformed. There was no relationship between the octanol/water partition coefficient and biotransformation of the PCDDs and PCDFs. No limitation of uptake for higher chlorinated PCDD and PCDF congeners was found.

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