In the present study, we first investigated the effect of paraquat (PQ) on nitric oxide (NO) biosynthesis in cultured A549 lung adenocarcinoma cells. A combination of PQ with cytokines synergistically increased the amount of nitrite, as an index of NOS activity, which paralleled the expression of inducible NO synthase (iNOS) mRNA.In addition, this endogenous NO showed a protective effect against PQ poisoning, while larger exogenous amounts of NO stimulated cell injury, probably because of interaction with superoxide derived from PQ.Next, using RT-PCR,we studied the effect of sub-lethal and lethal paraquat exposure on iNOS and interleukin (IL)-1beta gene expression in rat liver, kidney and lung. The iNOS signals in kidney and lung were unaffected, but a marked suppression and then a significant stimulation of the signals was observed in liver exposed to a lethal dose. The IL-1beta signal in liver, however, decreased throughout the experiment, suggesting that the changes in liver iNOS expression exposed to a lethal dose was not relevant to IL-1beta. In the lethal dose group, the IL-1beta signal in kidney reached a peak at 1 hr, declining by 3 hr, whereas that in lung increased throughout the experiment. These results demonstrate that PQ has a direct biphasic effect on iNOS expression in liver in vivo, thus providing evidence of possible involvement of nitric oxide generation in PQ-induced injury. On the other hand, they also indicate that the lung, the most severely affected organ, may be regulated by IL-1beta or other proinflammatory cytokines and not by NO.