Two rapeseed meals (RM1 and RM2), containing glucosinolates at a concentration of 26 and 40 mumol/g, respectively, were incorporated at increasing levels (10, 20, and 30% for RM1 and 30 and 50% for RM2) in diets of juvenile rainbow trout. Disturbances in the thyroid axis appeared after 14 days of feeding (with a dietary incorporation level of 10%). The dietary supplementation with T-3 or iodine induced an increase in plasma T-3 levels, compared to that in fish fed the RM diets, and reduced the deleterious effect of RM on growth. When trout were reared in seawater, there was also a slight increase in thyroid hormone levels. TSH treatment had no effect on the thyroid hormone plasma levels. The incorporation of 30% of RM1, which induced a lower dietary content of toxic compounds than RM2, led to a rapid decrease of plasma T-4 and T-3 levels, but growth was affected only after 6 months of feeding. During these studies, the deiodinase activities responded in a complex manner to restore plasma and tissue levels of T-3.