Effects of swimming exercise on the expression of estrogen receptor 1 vitellogenin1 and vitellogenin2. Relative expression of estrogen receptor 1, vitellogenin1 and vitellogenin2 (fold change fc of swimmers vs. resters) after a) swimming for 1.5 and 3 months (-CPE), and b) after subsequent stimulation with three weekly CPE injections (+CPE). Asterisks indicate a significant difference (P < 0.05) in ng mRNA of a swim group vs. the rest group. Expression of all genes is reduced after swimming 1.5 months and more after swimming 3 months vs. expression in the resting eels (fc<1). CPE treatment compensates for these differences equalizing the initial differences between resting and swimming (fc>1).