NOS-II gene expression can be induced in different cells by combinations of bacterial endotoxin and cytokines. However, NOS-II gene is expressed constitutively in certain human cancer cells such as colorectal adenocarcinoma cells (SW480) and melanoma cells (G361). These cancer cell lines express NOS-II mRNA constitutively at high (in case of SW480) and low (G361) levels. We have confirmed that one type of transcript is produced in the two cell lines, using RT-PCR analysis. The 5'- and 3'- RACE (rapid amplification of cDNA ends) products were compared with those from human tissues which showed differential expression pattern in the mRNA size. Two cell lines were stimulated with lipopolysaccharide (LPS), interferon gamma (IFN-gamma), tumor necrosis factor alpha (TNF-alpha), and interleukin IL-1). We have observed the presence of iNOS mRNA in the absence of measurable nitrite production in those cells. The discrepancy between the presence of NOS-II mRNA and the lack of nitrite production suggests that NOS-II gene is not controlled solely by transcription, but other post translation modification may be involved. These results suggest that some cancer cells may express NOS II gene constitutively at high levels, albeit under different regulatory control than stimulated-immune cells. Thus NO may have an unique function certain type of cancers.