Leishmaniasis is a group of diseases with a large spectrum of clinical manifestations caused by protozoans of the genus Leishmania. Here we demonstrate the leishmanicidal activity of the essential oil of Ocimum gratissimum as well as its main constituent, eugenol. The eugenol-rich essential oil of O. gratissimum progressively inhibited Leishmania amazonensis growth at concentrations ranging from 100 to 1000 microg/ml. The IC50 (sub-inhibitory concentration) of the essential oil for promastigotes and amastigotes were respectively 135 and 100 microg/ml and the IC50 of eugenol was 80 microg/ml for promastigote forms. L. amazonensis exposed to essential oil at concentrations corresponding to IC50 for promastigotes and for amastigotes underwent considerable ultrastructural alterations, as shown by transmission electron microscopy. Two or more nuclei or flagella were observed in 31% and 23.3% of treated amastigote and promastigote forms, respectively, suggesting interference in cell division. Considerable mitochondrial swelling was observed in essential oil-treated promastigotes and amastigotes, which had the inner mitochondrial membrane altered, with a significant increase in the number of cristae; in some amastigotes the mitochondrial matrix became less electron-dense. The minimum inhibitory concentration for both promastigotes and amastigotes was 150 microg/ml. Pretreatment of mouse peritoneal macrophages with 100 and 150 microg/ml essential oil reduced the indices of association between promastigotes and the macrophages, followed by increased in nitric oxide production by the infected macrophages. The essential oil showed no cytototoxic effects against mammalian cells. This set of results suggests that O. gratissimum essential oil and its compounds could be used as sources for new antileishmanial drugs.