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Ten Starling heart-lung preparations were given levarterenol, either as a single 5 µg dose or as a 2-minute infusion (5 µg/min.), while cardiac output and blood pressure were measured. A repeat dose 15–20 minutes after administration of 10 mg of hydrocortisone evoked a quantitatively similar response. Failure to observe potentiation in normal heart-lung preparations led to the use of bilaterally adrenalectomized dogs (maintained until 48 hr. prior to testing on cortisone acetate or DCA in oil), both as blood donor and preparation animal. In four such preparations similarly tested, no potentiation was observed. The blood pressure response to levarterenol was next tested in the intact dog. Two dogs were tested under local anesthesia with single 30-µg doses, and a third was tested with an infusion (5 µg/min.) while under light Nembutal anesthesia. Thirteen adrenalectomized dogs (maintained until 24 hr. prior to testing only on DCA in oil) were further tested under various conditions: either awake or anesthetized (either ventilated with 100% oxygen, or breathing spontaneously); either with a single dose (1–30 µg) or with a constant infusion (30–50 µg/kg/hr.). The response of all animals to levarterenol was unchanged after the administration of hydrocortisone or of aldosterone. It was further noted that all animals maintained a constant blood pressure response during the 10–90-minute period of levarterenol infusion before administration of hydrocortisone.