1. Exposure of tissues to endotoxin (LPS) and/or cytokines leads to the induction of both inducible nitric oxide synthase (iNOS) and cyclo-oxygenase-2 (COX-2). It has previously been reported that there is 'cross-talk' between these two systems. However, such previous studies have been limited by the availability of highly selective inhibitors. Here we have investigated the interactions between iNOS and COX-2 in vivo using 1400W, an iNOS-selective inhibitor, and celecoxib, a COX-2-selective inhibitor. 2. Infusion of LPS to rats for 6 h caused a time-dependent increase in the plasma concentrations of 6 keto-prostaglandin F1alpha (6 keto-PGF1alpha) and nitrite/nitrate (NO2/NO3), consistent with the induction of iNOS and COX-2. Bolus injection of arachidonic acid (AA) at t=6 h resulted in a further increase of circulating levels of 6 keto-PGF1alpha in LPS-treated animals. 3. Treatment of rats with 1400W or the non-selective NOS inhibitor N(G)-monomethyl-L-arginine (L-NMMA) inhibited the increase in plasma NO2/NO3 but were both without effect on the plasma concentration of 6 keto-PGF1alpha before or after AA. 4. Treatment with the non-steroidal anti-inflammatory drugs (NSAIDs), A771726 or diclofenac, or with celecoxib significantly reduced the increase in circulating 6 keto-PGF1alpha caused by LPS, and the large increase in 6 keto-PGF1alpha following injection of AA. None of the COX inhibitors affected the increase in plasma NO2/NO3. Dexamethasone, however, significantly inhibited both the increase in 6 keto-PGF1alpha and the increase in NO2/NO3. 5. In conclusion, the use of selective inhibitors does not support the concept of cross talk in vivo between iNOS and COX-2.