Forty-nine thyrotoxic patients prepared for partial thyroidectomy with the beta-adrenoceptor blocking drug, propranolol, and iodine are compared with 42 patients prepared with carbimazole and iodine. The age and sex distribution of the two groups were comparable, but patients with obstructive airways disease and possible cardiac insufficiency were excluded from preparation with propranolol. The mean duration of preoperative treatment with propranolol was 40 days, compared with 89 days for carbimazole. Propranolol treated patients had lower pulse rates before and after operation. The serum PB(127)I values immediately before and after operation were higher in the propranolol group than in the carbimazole group, but were the same in both groups one and four months after operation. The incidence of hypothyroidism at one year after operation was 30% in the carbimazole prepared patients and 31% in the propranolol patients. Serum calcium levels were higher in the propranolol group at the time of operation. No adverse effects from the use of propranolol and at operation the thyroid gland prepared with propranolol was firmer, less friable, more easily mobilised and less likely to bleed than the gland prepared with carbimazole. There is, consequently, less risk of damage to the parathyroid glands and recurrent laryngeal nerves. However, the basal metabolic rate remains high on propranolol therapy and very careful supervision is advised.