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Summary1. The ,8-adrenoceptor blocking properties of (± )-1-(2-acetyl-4-n-butyramidophenoxy)-2-hydroxy-3-isopropylaminopropane hydrochloride (M&B 17,803A) have been compared with those of practolol and propranolol in the guinea-pig. cat and dog. 2. Following either intravenous or oral administration in the cat or dog, M&B 17,803A and practolol had similar potency in antagonizing isoprenalineinduced tachycardia and both showed cardioselectivity, but both were less potent than propranolol. 3. M&B 17,803A and practolol had approximately one hundredth the intravenous potency of propranolol in increasing the severity of anaphylactic bronchospasm in the conscious sensitized guinea-pig. 4. M&B 17,803A possessed less marked intrinsic sympathomimetic activity than practolol but, like propranolol, it had significant local anaesthetic properties and increased the refractory period of rabbit isolated atria.

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