Systemic hypotension during sepsis is thought to be due to nitric oxide (NO) overproduction, but it may also be due to acidosis. We evaluated in healthy rats the consequences of acid infusion on NO and blood pressure. Sprague-Dawley rats were anesthetized, and ventilated with room air. The animals were randomized into four groups. Group 1 (C, n = 10) received only normal saline at rates comparable to the other groups. Group 2 (A1, n = 10) received hydrochloric acid at 0.162 mmol in the first 15 to 30 min, followed by a continuous infusion of 0.058 mmol/h for 5 h. Group 3 (AG + A1, n  = 6) was pretreated with aminoguanidine (AG, 50 mg/kg), and HCl was infused as above. Group 4 (A2, n = 7) received HCl at twice the rate used in A1. Nitric oxide concentration in the exhaled gas (ENO), blood gases, and mean arterial pressure were measured every 30 min. Acid infusion in A1 caused the pH to fall gradually from 7.43 ± 0.01 to 7.13 ± 0.05. This moderate decrease in pH was associated with a marked increase in ENO (1.6 ± 0.3 to 114.2 ± 22.3 ppb), an increase in plasma nitrite/nitrate (17.3 ± 3.7 to 35.2 ± 4.3 μ M), and a significant decrease in blood pressure (110.5 ± 6.3 to 63.3 ± 15.0 mm Hg). Furthermore, acidosis caused lung inflammation, as suggested by the increase in lung myeloperoxidase activity (282.2 ± 24.7 to 679.3 ± 57.3 U/min/g) and lung injury score (1.7 ± 0.2 to 3.5 ± 0.6). Acidosis after AG pretreatment was associated with a similar change in pH, but the increase in ENO, nitrite/nitrate, and systemic hypotension were prevented. Furthermore, lung injury was attenuated by AG, as suggested by a lower myeloperoxidase activity, though lung injury score was not altered. In this model, moderate acidosis causes increases in NO, hypotension, and lung inflammation. Lung inflammation and injury are due in part to acidosis and NO production. This is the first report to show a direct effect of chronic acidosis on NO production and lung injury. These results have profound implications on the role of acidosis on NO production and lung injury during sepsis.