Abstract 1. 1. Juvenile slow-growing European eels ( Anguilla anguilla) receiving a diet supplemented with 1, 5, 10 and 15 mg/kg 17-β-estradiol (ES) and 17-α-methyltestosterone (MT), were grown and studied for one and a half months in fresh water at a temperature of 23°C. 2. 2. At the end of the experimental period the mean weight of juvenile eels (elvers) which received hormones (MT or ES) were higher than the control, however, there was a significant difference ( P < 0.05) by t-test) only in elvers receiving 1, 5, 10, 15 mg/kg MT or 15 mg/kg ES in their diet. 3. 3. The growth equations were linear in the elvers receiving MT in the diet and were exponential in the elvers receiving ES in the diet. 4. 4. The percentage of protein retention (PPV), and energy retention (PEV) had negative linear correlations to MT concentration and a positive linear correlation to ES concentrations.