The toxins released by Bacillus anthracis are major determinants of its virulence, and have been linked to the symptoms noted during anthrax disease. These include pulmonary edema, hemorrhage, circulatory shock, and death. One of the toxins, lethal toxin (LT), produces these effects in mammalian models. Recent research has indicated that the profound increases in vascular permeability caused by LT are concomitant with the suppression of host immune responses rather than inflammation, making anthrax pathogenesis unique compared to many other microbes and viruses. LT cleaves and inactivates host kinases of the MKK/MEK family. These kinases regulate key points of cell function within signaling cascades; however no direct relationship between MKK inhibition and toxin effects has been demonstrated in vivo. The wide diversity of cellular processes that involve MKKs has made it difficult to dissect how their disruption might perturb vascular function, and little is known about specific cellular pathways involved in LT-induced pathologies. The discovery of the cellular anthrax toxin receptors in 2001 and 2003 prompted the first direct studies describing LT effects on endothelial cells, as each receptor is expressed during blood vessel formation. However, in vitro approaches failed to provide consistent results, and early consequences of LT action on the vasculature are difficult to investigate in mammalian models. To address these limitations, the zebrafish vascular model for toxin research was developed. Using this model, it was possible to distinguish LT effects from those of other pathogenic agents, while directly observing the biological consequences of LT action on endothelial cells within vessels in vivo. This approach revealed that LT can increase vascular permeability without inducing cell death. Furthermore, LT-specific phenotypes could be reproduced with a small molecule inhibitor targeting just two of its substrates: MKK1/2. Subsequent studies demonstrated that constitutive MKK1 activation rescued LT effects in transgenic zebrafish embryos, suggesting an important role for the disruption of MKK1/2 signaling in the toxin's vascular dysfunction. These findings facilitate the characterization of what cellular pathways are involved in LT-induced leakage. Additionally, the tools and approaches developed in this dissertation may be applied to the study of other pathogens, cancer biology, and development.