METHODS

Twelve conditioned, mongrel dogs weighing from 14 to 23 kg had an electromagnetic flow transducer(Biotronex series 5000 or In Vivo Metric series FT-Q) implanted around the ascending aorta 7-9 days prior to the experiment. On the day of experiment the animal was sedated with morphine sulfate (2 mg/kg) and then anesthetized with a-chloralose (60 mg/kg) and urethan (400 “g/kg). Supplemental anesthesia was given as needed using one-fourth the initial amount. The animals were heparinized and both carotid sinuses were functionally isolated using a modified Moissejeff technique(17). The internal carotid, occipital, and any small arterial branches were ligated. Bulldog clamps were used to occlude the external carotid arteries during the experimental run. A four-way glass junction was connected to the cephalad segment of both common carotid arteries, the caudal portion of the left common carotid artery, and a pressure-generating system. This system, which has been described previously (14), consisted of a comparator, a servo amplifier and motor-driven water-filled syringe pump (Harvard Apparatus Co.), and a pressure transmission chamber which was coupled to the pump and contained blood and water separated by a thin rubber membrane. The inputs to the comparator were an electrical