Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) ; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) ; Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) ; Processo FAPESP: 05/01550-8 ; Protozoans of the trypanosomatid family cause the neglected tropical diseases leishmaniasis and trypanosomiasis, for which few drugs are available. In this context our group has recently reported that the essential oil obtained by steam distillation of the fruits of Piper cubeba is active against Schistosoma mansoni. Therefore, we have investigated the in vitro effects of the essential oil against the trypomastigote and amastigote forms of Trypanosoma cruzi isolated from an LLCMK2 cell line culture and the promastigote forms of Leishmania amazonensis. The in vitro activity of the essential oil against trypomastigotes of T. cruzi increased upon rising concentrations, giving IC50 values of 45.5 and 87.9 mu gmL(-1) against trypomastigotes and amastigotes, respectively. The essential oil was not active against L. amazonensis, since it displayed lyses of only 24% at 400 mu gmL(-1), and an IC50 of 326.5 mu gmL(-1). Therefore, the essential oil should be further investigated to determine the compounds responsible for the observed activities, as well as its mechanism of action.