IV adrenaline increases coronary and cerebral perfusion pressures during cardiopulmonary resuscitation. We recently showed that endotracheal adrenaline can decrease blood pressure (BP), a detrimental effect presumably mediated by the beta 2-adrenergic receptor unopposed by alpha-adrenergic vasoconstriction. This prospective, randomized, laboratory comparison of endotracheal adrenaline (0.05 mg/kg diluted with normal saline to 10 mL total volume) with or without nonselective beta-blocker (propranolol) pretreatment was conducted in an attempt to clarify the mechanism of this BP decrease. Five mongrel dogs were given 0.05 mg/kg endotracheal adrenaline (diluted) or 0.05 mg/kg endotracheal adrenaline followed by an IV propranolol (0.1 mg/kg) pretreatment. Each dog served as its own control (10 mL of normal saline administered endotracheally) and received each regimen at least one week apart. Endotracheal adrenaline given after the propranolol pretreatment produced an increase in systolic, diastolic, and mean arterial BPs, from 165/110 mm Hg (mean 128 mm Hg) to 177.5/125 mm Hg (mean 142.5 mm Hg), respectively, as opposed to the hypotensive effect of isolated endotracheal adrenaline (P < 0.03). Thus, endotracheal adrenaline was associated with predominantly beta-adrenergic-mediated effects, causing hypotension via peripheral vasodilatation unopposed by alpha-adrenergic vasoconstriction. The search for the optimal dose of endotracheal adrenaline should be aimed at achieving the higher alpha-adrenergic vasoconstrictive threshold.