Weekly injections of human chorionic gonadotropin (hCG; 1·5 IU g 1 W) induced spermiation in 87-100% farmed male European eels Anguilla anguilla over a 10-week period, producing 3 ml (100 g) 1 milt volume, at 1·4 10 9 spermatozoa ml 1 sperm mobility peaked in ninth week of treatment with 97% of males with motile sperm. Gonadotropin-treated males showed earlier but similar morphological changes to control males. Whereas higher sperm density was obtained 6 h after hCG administration, higher percentage of motile cells and longer sperm mobility were observed 24 h after the treatment indicating that this is the optimum time to obtain good sperm after the hormonal induction. Several monounsaturated fatty acids in sperm increased significantly from week 5 to 9 of treatment (when highest volume, density and mobility of milt were recorded), whereas eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and total n-3 polyunsaturated fatty acids (PUFAs) were significantly reduced. Fatty acid levels returned to initial values after 13 weeks of treatment. These results are in agreement with the reduction of n-3 PUFAs recently observed in the ovary of female Japanese eel during artificial maturation, and an attempt is made to suggest a physiological explanation of its possible relation with sperm quality. 2000 The Fisheries Society of the British Isles