Brassica vegetables are the major source of glucosinolates in the human diet. Certain glucosinolates are readily converted into goitrogenic species, notably 5-vinyloxazolidine-2-thione and thiocyanate ion. The effect of dietary Brussels sprouts, a particularly rich source of such glucosinolates, on thyroid function has been examined. Inclusion of cooked Brussels sprouts (150 g daily for 4 weeks) into a normal diet of 10 volunteer subjects had no effect on thyroid function as determined by measurement of thyrotrophic hormone, thyroxine and tri-iodothyronine even though the sprouts contained high concentrations (220 mg/100 g) of glucosinolates. In view of the reported antithyroid activity of 5-vinyloxazolidine-2-thione it is suggested that this lack of activity of cooked Brussels sprouts is due to inactivation during cooking of myrosinase, the specific glucosinolate-degrading enzyme.