The antagonism by propranolol and alpha-methylpropranolol (ICI 77602) of vascular and cardiac responses to isoprenaline in anaesthetized dogs.

1. In anaesthetized dogs, alpha-methylpropranolol was less potent than propranolol in antagonizing both vascular (hind limb perfusion pressure) and cardiac heart rate) responses to isoprenaline. 2. alpha-Methylpropranolol was more potent in antagonizing vascular than cardiac responses to isoprenaline, but this selectivity was no greater than that seen also with propranolol. 3. Isoprenaline sensitivity was greater in the hind limb than the heart and vascular-selective antagonism was more pronounced in those dogs in which this differential sensitivity was the greatest. 4. Introduction of an alpha-methyl group into propranolol decreases its beta-adrenoceptor antagonist potency but does not enhance vascular selectivity.