Recently, our group demonstrated that mouse lesions infected with Leishmania amazonensis are hypoxic. Evidence indicates the negative impact of hypoxia on the efficacy of a variety of chemotherapeutic agents against tumors, fungi, bacteria, and malaria parasites. In the present study, comparison of the effect of antileishmanial drugs on L. amazonensis-infected macrophages under normoxic and hypoxic conditions was performed. We compared the effect of 5% oxygen tension with a tension of 21% oxygen on peritoneal murine macrophage cultures infected with the parasite and treated with glucantime, amphotericin B, or miltefosine. Analysis of the infection index (percentage of infected macrophages × number of amastigotes per macrophage), dosedependent efficacy of drugs, and IC50 values demonstrated that hypoxia conferred a small, but significant, resistance to all 3 antileishmanial drugs. The present finding suggests that in vitro assays under hypoxia should not be neglected in drug studies.