Haemodynamic interactions between alpha- and beta-adrenoceptor antagonists in conscious or anaesthetized rats

1. The alpha-adrenoceptor antagonist, phentolamine, was administered intravenously, to conscious or anaesthetized rats, before or after the beta-adrenoceptor antagonist, propanolol. 2. The bradycardia which followed the combined administration of propranolol and phentolamine exceeded that induced by propranolol alone, yet the hypotensive effect of phentolamine was attenuated by propranolol. 3. The results are compatible with the concept that a significant part of the hypotensive effect of phentolamine is mediated by stimulation of vascular beta-adrenoceptors.