The influence of β-adrenoceptor antagonists on serum TSH level (supersensitive method) and thyroid volume has not previously been studied. Thirty-two young non-smoking males were treated for 3 weeks with either atenolol 50 mg (b.i.d.), metoprolol 100 mg (b.i.d.) or propranolol 80 mg (b.i.d.) in a placebo controlled study. After 1 week, median serum TSH level increased in the atenolol (from 1.76 (range: 0.96-4.04) to 2.25 (range: 1.11-4.22) mU/l, P < 0.05) and propranolol (from 1.91 (range: 0.90-3.83) to 2.44 (range: 0.75-6.30) mU/l, P < 0.05) treated groups. After 3 weeks, median serum TSH reached pretreatment level in the atenolol treated, whereas median serum TSH decreased compared to pretreatment values in the propranolol treated (1.68 (range: 0.68-3.62) mU/l, P < 0.05). Except for a slight increase in the atenolol treated group, no changes in median thyroid volume was seen after 3 weeks. The changes in serum TSH or thyroid volume were not related to changes in the concentrations of thyroid hormones, or of a magnitude likely to interfere with the clinical evaluation of thyroid function.