Because of valve malfunction and cerebral embolism reported with use of the total artificial heart, the descending aorta was reexamined as a site for a valveless counterpulsation left ventricular assist pump. The pump was introduced into the thoracic aorta at the level of the eight thoracic vertebra in 11 canine right-heart bypass preparations. Heart rate (110 beats per minute), stroke volume (20 ml), and mean arterial pressure (80 or 100 mm Hg) were precisely controlled. The pump ejection volume was adjusted to equal 1 time or 2 times the stroke volume of the dog. A 20-ml ejection led to an 11% reduction in left atrial pressure, an 11% reduction in peak left ventricular pressure, and a 20% reduction in tension time index (all, p less than .05 at a mean arterial pressure of 80 mm Hg). Systolic shortening (ultrasonic crystals) increased by 5% (p less than .05 at a mean arterial pressure of 80 mm Hg). A 40-ml pump ejection led to an 11% reduction in mean left atrial pressure, a 17% reduction in peak left ventricular pressure, and a 21% reduction in tension time index (all, p less than .05 at a mean arterial pressure of 80 mm Hg). The systolic shortening increased by 15% (p less than .05). To assess the relative risk of cerebral embolism, albumin spheres (40 to 60 micrograms) labeled with technetium-99 were injected into the pump chamber in 5 dogs (40-ml ejection, 2.1 +/- 0.1 times the dog's stroke volume).(ABSTRACT TRUNCATED AT 250 WORDS)