The transformation of yellow eel into silver eel is called 'silvering', and takes place prior to migration. This is the first study to provide hormonal profiles of European eel (Anguilla anguilla L.) during silvering. This transformation occurs in association with hormonal surges of testosterone (T) and estradiol (E2) but not with thyroid hormones (TH) and growth hormone (GH) which have a maximum activity in spring and a minimum activity in summer and autumn. It is therefore suggested THs and GH are not important for eel gonadal development in the autumn. Based on PCA analysis with physiological, morphological and endocrinological parameters it is concluded that the transition is gradual and that eels go through several stages.