The objective of the study was to determine the composition of fatty acids in the fat of Atlantic herrings (Clupea harengus) after each subsequent stage of producing cold marinades thereof, and, also, during the period of storage. The study was performed using frozen fillets made of Atlantic herring fish (n = 320) netted in the FAO 27 fishing zone in the North Sea. Deep-frozen (-18 ° C), 15 – 20 kg blocks of the fish netted were delivered to a manufacturing plant. According to the Polish Standards, the fish fillets represented an H assortment, i.e. 10 – 16 fillets equalled 1 kg of raw material. Two experiments were carried out; each one was divided into 4 stages. The first two stages were the production stages; they were performed at the processing plant. The other two were incorporated to determine the changes in the fat of fish analyzed during their storage. In stage 1, when the fish fillets were initially mixed with the marinating brine, two methods applied by the manufacturing plant were studied. Method I was a traditional method: components of the marinating brine diffused into the raw material during the marinating bath in a pool. In Method II, a special rotating drum was used: the raw material was in the drum and the marinating brine was supplied thereto by a pipeline. The process of marinating did not cause any significant changes in the percentage content of individual fatty acid groups. The monoenic acids prevailed, next, the saturated acids came followed by the polienic acids (MUFA>SFA>PUFA). Significant (p < 0.05) changes were related only with the decreases in the level of PUFA and in the n-3/n-6 ratio (p < 0.01) in stage II in the two experiments. On this basis, it can be concluded that the use of rotating drum in order to better mix the raw material with the marinating brine did not contribute to the decreased in the PUFA level found in the marinated fillets. The traditionally marinated fillets were characterized by higher contents of EPA and DHA acids compared to the fillets marinated in the rotating drum. During the 30 and 60 day storage periods, the level of long-fatty n-3 PUFA acids in the marinating fillets was still high and did not differ significantly from the levels thereof in raw fillets.