Propranolol alone was used to prepare 20 thyrotoxic patients, 19 women and 1 man, for subtotal thyroidectomy. Serum thyroxine (T4) and triiodothyronine (T3) concentrations were measured immediately before, at several stages during and after the surgical procedure. As judged primarily by the cardiovascular response, an average of 80 mg (range 40 to 120 mg) of propranolol qid for 8 days (range 3 to 18 days) was required to prepare the patients. During the various stages of surgical removal there was no change from the initial mean (+/- SEM) T4 concentration of 25.0 +/- 2.5 microgram/dl (321.8 +/- 32.2 nmol/l) or T3 concentration of 4.2 +/- 0.6 microgram/l (6.45 +/- 0.92 nmol/l) (P greater than 0.2). At discharge on the fifth postoperative day values were significantly lower, 12.9 +/- 1.5 microgram/dl (166.0 +/- 19.3 nmol/l) and 1.9 +/- 0.2 microgram/l (2.9 +/- 0.31 nmol/l), respectively (P less than 0.001). There were no operative complications but four patients had transient hypoparathyroidism. After 1 year 2 of 18 patients had permanent hypoparathyroidism and 4 of the 18 followed up for 1 year had permanent hypothyroidism requiring thyroid hormone replacement. There was no instance of recurrent thyrotoxicosis. The authors conclude that during surgical manipulation of the gland no release of thyroid hormones into the circulation was detected and that, using propranolol as the sole agent, thyrotoxic patients can be rapidly and safely prepared for subtotal thyroidectomy.