Local blood-brain barrier (BBB) opening is an advantageous approach for targeted drug delivery to the brain. Recently, it has been shown that focused ultrasound (US) exposures (sonications), when applied in the presence of preformed gas bubbles, caused magnetic-resonance (MR) proven reversible opening of the BBB in targeted locations. The cellular mechanisms of such transient barrier disruption are largely unknown. We investigated US-induced changes in endothelial cell fine morphology that resulted in the BBB opening in rabbits. To obtain evidence for the passage of blood-borne macromolecules through the opened transvascular routes, an immunocytochemical procedure for endogenous immunoglobulinG (IgG) was performed, in addition to the routine electron microscopy. An increased number of vesicles and vacuoles, fenestration and channel formation, as well as opening of some tight junctions, were seen in capillaries after low-power (0.55 W) sonication. Immunosignals presented in some of the vesicles and vacuoles, in the cytoplasmic channels and, so rarely, in intercellular clefts; immunosignals could also be seen in neuropil around the blood vessels. Damage to the cellular ultrastructure was not seen in these areas. However, cell destruction and leakage of IgG through defects of the endothelial lining took place at 3 W sonications. The data reveals that several mechanisms of transcapillary passage are possible after such sonications: 1. transcytosis; 2. endothelial cell cytoplasmic openings--fenestration and channel formation; 3. opening of a part of tight junctions; and 4. free passage through the injured endothelium (with the higher power sonications). These findings could be considered in further development of the strategy for drug delivery to brain parenchyma.