Twenty-one hyperthyroid patients participated in an 8-week double-blind crossover trial of propranolol and practolol, and the effecte of these drugs on the clinical and metabolic features of the disease were studied. Propranolol was marginally more effective than practolol, as measured by the hyperthyroid diagnostic index and anxiety scale. Propranolol produced a significant reduction in the serum concentration ratio of tri-iodothyronine to thyroxine, compatible with partial inhibition of peripheral deiodination of thyroxine. Adverse reactions occurred more frequently with propranolol than with practolol. In veiw of the efficacy of practoloo, further trials in hyperthyroid patients of newer beta1-adrenoceptor antagonists, preferably without partial agonist activity, are indicated.