Drug discovery from plants plays an important role in the pharmaceutical therapy field and the alkaloids lycorine and candimine are candidates for this purpose. Trichomonas vaginalis is a parasite that infects the human urogenital tract and causes trichomonosis, the most prevalent non-viral sexually transmitted disease. Ecto-nucleotidases including nucleoside triphosphate diphosphohydrolase (NTPDase) members, which hydrolyses extracellular ATP (adenosine triphosphate) and ADP (adenosine diphosphate), and ecto-5'-nucleotidase, which hydrolyses AMP (adenosine monophosphate), have been characterized in T. vaginalis. Because purine nucleotides are released from cells under physiological and stress conditions, the goal of this study was to evaluate the effect of lycorine and candimine on T. vaginalis NTPDase and ecto-5'-nculeotidase activities. The alkaloids (50 to 250microM) were tested against both long-term-grown and clinical isolates. Specific enzymatic activities were expressed as nmolPi released/min/mg protein. The effect of both alkaloids at NTPDase A and B expression levels was investigated. When the alkaloids were added directly to the reaction mixture, no effect on ATP, ADP or AMP hydrolysis was observed. NTPDase and ecto-5'-nucleotidase activities were strongly inhibited by candimine and lycorine on 24h-treated parasites. This effect was abolished when 24-treated parasites were innoculated in a culture medium without alkaloid. Transcript levels of NTPDase A or B were not altered by the alkaloids. Considering the cytotoxic and proinflammatory roles of ATP besides the anti-inflammatory effects of adenosine, the regulation of extracellular nucleotide levels could be relevant in increasing susceptibility of T. vaginalis to host immune response in the presence of lycorine and candimine.