1. Cardiovascular events and outcome in septic shock may be predicted by monitoring the fall in intramural pH (pHi), as an index of splanchnic perfusion and mucosal ischaemia. In the present study, a small animal model for monitoring the changes of gastric pHi or intramucosal [H+] following challenge with the endotoxin lipopolysaccharide (LPS) was developed in the rat. The role of nitric oxide (NO) in these events in this model was evaluated using the non-selective NO synthase (NOS) inhibitors N(G)-nitro-L-arginine methyl ester (L-NAME) and N(G)-monomethyl-L-arginine (L-NMMA). 2. The pHi and intramucosal [H+] were evaluated in omeprazole-pretreated rats (30 mg/kg, i.p.) using the Henderson equation after estimating the PCO2 and the bicarbonate concentration in gastric wall. To measure gastric wall PCO2, the oesophagus was intubated and the pylorus ligated. The PCO2 was measured by a blood gas analyser in 2 mL saline instilled for 30 min in the gastric lumen to equilibrate with the gastric wall. The pHi was measured under basal conditions and 3 and 5 h after LPS (3 mg/kg) administration. Separate groups received treatment with L-NMMA (25-50 mg/kg) or L-NAME concomitantly or 2.5 h after administration of LPS. 3. Intravenous administration of Escherichia coli LPS provoked a significant fall in gastric pHi from 7.37 to 7.18 (median values; n =10-19) determined after 5 h. In groups treated concurrently with LPS and L-NAME (5 mg/kg; n = 19), there was a similar increase in intramucosal [H+] as that induced by LPS alone (n = 15) in those animals that survived. In contrast, L-NAME (5 mg/kg; n = 12), given 2.5 h after LPS challenge, at a time at which inducible NOS is known to be significantly expressed, prevented the increase in intramucosal [H+] at 3 and 5 h after LPS challenge. Similarly, L-NMMA (25-50 mg/kg; n = 23), given 2.5 h after LPS challenge, dose-dependently inhibited the increase in intramucosal [H+] at 3 and 5 h. 4. In conclusion, these findings indicate that this rat model could be useful in exploring the pathophysiology of acute endotoxin shock. Delayed administration of L-NAME and L-NMMA abolished the increase in gastric intramucosal [H+], supporting the involvement of excess NO in the tissue dysfunction associated with endotoxin shock. This suggests the potential value of this small animal model in evaluating the therapeutic activity of novel agents for use in septic shock.