Leishmania spp. are an important pathogen of humans and other vertebrate animals, with significant global prevalence and impact on human and animal health. Leishmania chagasi is one species that causes visceral leishmaniasis, a potentially fatal disease of humans: annual incidence is about 0.5 mil. These insect-vector borne protozoan pathogens undergo complex development within the sand fly as they progress from morphological forms having low infectivity (to vertebrates) to a form that has high infectivity. Repeated serial passage of promastigote cultures results in cell populations that exhibit perturbations in developmental progression, in expression levels of surface macromolecules (Major Surface Protease, MSP, and Promastigote Surface Antigen, PSA), and in virulence properties including resistance to serum lysis. One area of study presented within this thesis is a determination of the temporal abundance of morphologically distinct parasite stages that appear within axenic in vitro cultures of L. chagasi parasites. The principal finding is that nectomonad promastigotes predominate, while metacyclic promastigotes diminish, in cultures that have been serially passaged more than seven times and that are at stationary growth phase. A second, related area of study also presented within this thesis was designed to optimize the animal-model culture system used to produce and propagate infectious parasites. The principal finding on this was that parasite development and characteristics were equivalent in cell cultures regardless of whether cultures were initiated using never-stored or cryopreserved cells. The second study also validated the use of saphenous vein inoculation for the inoculation/infection of L. chagasi parasites into hamsters, a common animal model of visceral leishmaniasis; saphenous vein inoculation is considered a more humane procedure than the alternative and commonly used cardiac puncture method. As a result, the second study increases the utility and efficiency of the in vitro model of visceralizing Leishmania spp. and decreases the number of animals needed to maintain the animal model.