American eels Anguilla rostrata, collected from three distinct locations along the St Lawrence River (Lake St Lawrence, Quebec City and Kamouraska), were thought to consist entirely of out-migrating adults at Quebec City and Kamouraska and of both resident and migrants at Lake St Lawrence. The mean plasma levels of 17 -oestradiol closely paralleled the trend observed with gonadosomatic index (I G ) and oocyte development. The highest levels of 17 -oestradiol and I G were observed in Quebec City and Kamouraska, while the levels observed in the resident eels from Lake St Lawrence were very low. Generally, the concentration of total plasma non-esterified fatty acids (NEFA) appeared to follow the trend of 17 -oestradiol and I G and increased with sexual maturity. However, the most sexually mature group, eels from Kamouraska, had the lowest total plasma NEFA levels. The reason for the drop is not clear at present. In addition, a number of shifts in the plasma NEFA profiles of American eels did occur during their migration in a downstream direction, with several fatty acids either increasing or decreasing in relative abundance. The ratio of n3 : n6 fatty acids declined with increasing sexual maturity as most n3 fatty acids became less abundant. Although the correlation between gonadal development and the relative concentration of individual fatty acids was not always clear, both arachidonic acid (20 : 4n6) and docosahexaenoic acid (22 : 6n3) were significantly higher in the plasma of out-migrating adults than in resident yellow phase eels. 2001 The Fisheries Society of the British Isles