During malarial infection by Plasmodium falciparum, significant, amounts of hemoglobin are degraded by the intraerythrocytic parasite in a specialized food vacuole. The released amino acids are utilized to support growth, but heme is mostly detoxified to hemozoin in the food vacuole. However, it is unclear how much heme partitions to the parasite's cytosol, and the role it plays in parasite physiology. Furthermore, chloroquine, a commonly used anti-malarial drug has been suggested to increase cytosolic heme flux, but this has not been proven. We will address these issues by using a functional approach rather than mass balance measurements that are prone to large errors in biological systems. Our goals are to (1) develop high affinity RNA aptamers to heme using SELEX, and, (2) introduce them into P. falciparum, and determine their impact on parasite survival in the presence and absence of chloroquine. The aptamers are expected to be cytosolic given their negative charge, and should not interfere with heme polymerization in the food vacuole, thus providing a tool to selectively investigate the heme pool in the cytosol. These studies are important to advance our understanding of how P. falciparum disposes and/or utilizes host-derived heme, and to specifically investigate a potential mechanism by which the commonly used anti-malarial drugs exert their toxicity. Resistance to chloroquine is becoming increasingly widespread, and occurs via efflux mechanisms that prevent drug accumulation within the parasite. If parasite survival is intricately associated with the status of a physiologic cytosolic heme pool, this may represent another drug development target that circumvents the known resistance mechanism.