1. The influence of adrenergic receptor blocking drugs on the mouse thyroid gland maintained in vitro has been investigated.

2. Phentolamine, an α adrenergic blocking drug, and propranolol, a β blocking drug, inhibited the release of [131I]iodothyronines from pre-labelled mouse thyroids, which otherwise occurred when the glands were incubated in the presence of thyroid stimulating hormone, long acting thyroid stimulator, or cyclic 3′5′-adenosine monophosphate.

3. Evidence is presented to show that (a) the inhibition is not due to adrenergic blockade, (b) the effect cannot wholly be attributed to the prevention of adenyl cyclase activation, (c) the mechanism of action of the two drugs is dissimilar.

4. The observed clinical response in the treatment of thyrotoxicosis does not appear to be related to this antithyroid effect of propranolol.

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