An in vivo bolus generation system has provided information concerning acoustic cavitation in blood and associated bioeffects. Microbubble boluses produced in the canine abdominal aorta by tone bursts of 1.8 MHz focused ultrasound were detected in distal arteries using medical ultrasound scanners and a resonant bubble detector. At the maximum acoustic intensity, 19,000 W/cm[sup 2](I[inf SPTA]), 125--250 ms bursts generated contrast enhancement persisting several cardiac cycles. Intensities down to 3,700 W/cm[sup 2] yielded progressively weaker boluses while shorter and weaker boluses were produced with 64--0.5 ms bursts at the maximum acoustic intensity. There was a 50% occurrence of vascular wall effects (by histology) but no immediate mechanical failure. Oxygen ventilation has been shown to destabilize IV injected contrast agents and similarly, cavitation thresholds were elevated in the vena cava. Resonant bubble dectection indicated the presence of 7 (mu)m diameter bubbles but the bubble size range is unknown. These results provide insight into bubble production by acoustic cavitation but any bioeffects must be examined in the context of arterial bolus production where the risk of significant damage may be less than that of alternative procedures. [Work supported in part by USPHS 5 RO1 DK42290.] [sup a)]Current address: Acuson, Inc., Sunnyvale, CA.