With intense, focused ultrasound pulses it is possible to generate microbubble boluses in the arterial system with potential diagnostic and therapeutic uses. Arterial microbubbles have been generated recently in flowing whole blood in vitro with 725 kHz ultrasound passing through canine tissues simulating human transcutaneous generation. Similar generation was reported previously [J. A. Ivey et al., Ultrasound Med. Biol. 21, 757--767 (1995)] from interoperative, in vivo exposures at 1.8 MHz and at higher intensities. If biological effects can be controlled acceptably, knowledge of ultrasonic bioeffects will be enhanced. Short boluses of ultrasonically imageable, less than 40 micron bubbles, generated in selected arteries, might be usable for diagnosis and monitoring of those macrovascular and perfusion abnormalities which currently are evaluated more slowly and, presumably, expensively. Very sparse distributions of 30 to 40 micron bubbles should provide point beacons with which to refocus ultrasound beams for high resolution imaging and treatment through aberrating tissues including the skull. Identification of feeder arteries to and disruption of flow to therapeutic targets should be achievable to enhance chemical or ultrasonic therapy. [Work supported in part by USPHS grant No. 2RO1 DK 42290 from the Nat'l Inst. Diab., Digestive and Kidney Disease.] [sup a)]Current address: Acuson, Inc., Mountain View, CA.