The effect of propranolol on radioiodine utilization by the thyroid gland has been studied in 18 patients. All parameters of iodine uptake by the thyroid – initial 5–20 minutes after intravenous administration, 2, 4, 24 and 48 hours after oral administration were unaltered by propranolol administration. In three of four patients studied an increase in isotopically labelled serum PBI was observed after 8 days propranolol therapy.