Nitric oxide (NO) plays an important role as a mediator of the immune response to intracellular pathogens in mice; however discordant results were obtained in in vitro human cell lines experiments. Thus, we found that nitrite levels (nitric oxide derivatives) were increased in presence of Toxoplasma but not with IFN gamma plus LPS treatment during Toxoplasma infection of a human monocyte cell line THP1 and Griess assays confirmed that Toxoplasma alone has a nitrite production that was surprisingly increased by the most common inhibitor of nitric oxide synthase (NOS) in mammals. To look for genomic sequences that code for NOS gene in Toxoplasma, which could explain this production of NO derivatives, specific NOS motifs were sought by bioinformatics methods. A putative NOS motif sequence was found in one contig of the Toxoplasma genome (). Specific primers amplified a segment of 270 bp in RT-PCR assay, indicating that it is a transcription gene in the tachyzoite stage. Our results are the first description of the existence and transcription of a putative NOS DNA sequence in a pathogenic protozoan.