To examine the possible harmful effects of feeding Brussels sprouts to rats, groups of 10 male, weanling rats received the non-dehydrated vegetable in moist diets at levels providing 2.5-30% of the dry matter for 4 wk. A first study comprised test diets with 15 and 30% of the dry matter as uncooked or cooked Brussels sprouts and control diets without and with 0.2% potassium thiocyanate (KSCN) for comparison. The second study comprised diets with 0, 2.5, 5, 10 and 20% of the cooked vegetable and diets with 0 and 20% of the cooked vegetable with extra iodine. Diets with the uncooked vegetable contained considerably less intact glucosinolates than did diets with the cooked product, probably as a result of more extensive enzymatic degradation in the uncooked product. Growth depression and decreased food intake, not accompanied by decreased food efficiency, occurred in rats fed 10% or more dry matter as Brussels sprouts. These findings were less marked with the cooked than with the uncooked vegetable, probably because of unpalatability. Decreased levels of blood haemoglobin and plasma thyroxin were found with 15% or more Brussels sprouts. Prothrombin times were increased if 2.5% or more was fed. Thyroid stimulating hormone was increased by feeding potassium thiocyanate, but not by feeding the vegetable. Increased kidney weights and impaired kidney function not accompanied by microscopic renal changes were observed in rats fed 10% or more Brussels sprouts. Increased liver weights, which occurred from the 5% level, were accompanied by microscopic hepatic changes only at feeding levels from 10% of the cooked vegetable. 'Morphological activation' of the thyroid was increased with 10% or more of the cooked vegetable and with 0.2% KSCN. Iodine supplementation of the diets did not influence the results obtained with the vegetable. These studies indicated that 2.5% Brussels sprouts dry matter in the diet was not without effect, and that the thyroid characteristics were less sensitive to Brussels sprouts than were other criteria examined.