Background: We have previously demonstrated the ability of continuous wave and pulsed Doppler ultrasound (US) to increase the permeability of the arterial wall to echogenic liposomes (ELIPs). We have modeled this phenomenon in vitro using transwell endothelial cell monolayers. This study uses the latter to explore potential mechanisms of US-enhanced vascular permeability. Hypothesis: US facilitates endothelial cell permeability by radiation pressure, rather than cavitation effects, mediated by nitric oxide (NO) generation. Methods: Human umbilical vein endothelial cells (HUVEC) confluently grown in 24-well transwell insert plates were treated with TNF-α (20 ng/mL) and incubated overnight at 37°C. After washing with Dulbecco’s PBS (DPBS), cells were incubated with fluorescently (rhodamine) labeled ICAM-1-targeted echogenic liposomes (Ab-ELIP). Treatments included Ab-ELIP, nonspecific IgG ELIP, non-echogenic Ab-ELIP, and Ab-ELIP with 10 μM NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of endothelial nitric oxide synthase (eNOS), or 1H-[1 ,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 0.1 and 1.0 μM), an inhibitor of soluble guanylyl cyclase (sGC). Some plates were treated with continuous wave ultrasound (1 MHz, 2 W/cm 2 , 50% duty cycle for 1 minute). After 30 minutes, the insert plates were washed with DPBS, filled with endothelial cell medium and incubated at 37°C for 24 hours; 150 μL of lower well media were transferred to 96-well plates, then measured for fluorescence intensity with a microplate reader. Results: US treatment enhanced endothelial cell penetration of Ab-ELIP relative to non-US treatment (18 DOF, p=0.000012). Non-echogenic ELIP was as effective in this regard as echogenic ELIP (p=0.57), supporting the hypothesis that the ultrasound effects noted in these experiments were due to radiation pressure. L-NAME suppressed US-enhanced penetration of Ab-ELIP to non-US levels (p =0.14), indicating that US enhancement of Ab-ELIP penetration is mediated by NO generation. ODQ (0.1 μM) reduced the US penetration enhancement (p=0.0002), suggesting that the effect is at least partially mediated by NO-induced cGMP production. Conclusion: Both aspects of the hypothesis were supported by the experimental results.