Baltic herring (Clupea harengus) oil was extracted and fractionated. To examine the contribution to toxicity and biological effects of different halogenated organic pollutants, the herring oil and the fractions were mixed into pelleted food and given to Sprague-Dawley female rats at three levels, corresponding to a human intake of 1.6, 8.2 and 34.4 kg fish per week. Herring oil, its fractions, as well as liver tissues from exposed rats, were analyzed for: eight chlorinated biphenyls, all 2,3,7,8-substituted chlorinated dibenzo-p-dioxins and dibenzofurans, hexachlorocyclohexanes, hexachlorobenzene, 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane (DDT), DDT-metabolites, three brominated diphenylethers as well as extractable organically bound chlorine and halogenated fatty acids. A bioassay (EROD) was used for measuring the dioxin-like enzyme induction activity. Nordic Sea lodda (Mallotus villosus) oil was used as a nutritionally equivalent control, with much lower levels of halogenated organic pollutants. A full toxicological subchronic examination is reported in the following paper (Stern et al. 2002). In this study, we report that the fractionation procedure resulted in a substantial reduction of most of the pollutants in the triacylglycerol fraction, and a pronounced enrichment of most of the pollutants into the two other fractions. However, all contaminants were present at some levels in all of the fractions. The concentrations of organohalogens found in this study were representative for Baltic herring during the mid-1990s. Rat liver tissue showed similar residue patterns as the diet, with the exception of chlorinated dibenzo-p-dioxin and dibenzofuran congeners that had a higher liver retention than pesticides, chlorinated biphenyls and brominated diphenylethers.