['We tested the hypothesis that a sublethal dose of lipopolysaccharide in vivo induces in vitro atrial depression in rats via release of cyclooxygenase products. Male Sprague-Dawley rats (mean weight 366 +/- 9 [SE] g) were injected iv at time 0 with Escherichia coli lipopolysaccharide (1 mg/kg, n = 22) or saline (2.5 mL, n = 11). Some animals injected with lipopolysaccharide were pretreated 30 mins before with ibuprofen (15 mg/kg, n = 7). At time 2 hrs, the hearts were harvested and the atria were immersed in tissue baths and attached to force displacement transducers. Blood was collected for measurement of thromboxane B2 (TxB2) by radioimmunoassay. Force of contraction and rate in response to four different preloads were measured. Afterward, the preload was changed to 1 g, and force of contraction and rate were evaluated in response to graded doses of isoproterenol.', 'Force of contraction was significantly (p less than .05) lower at all preloads in animals given lipopolysaccharide. Pretreatment with ibuprofen did not prevent the depression in force of contraction but prevented the increase in TxB2 seen after lipopolysaccharide injection (p less than .05). The force of contraction and rate response to isoproterenol were similar across groups.'] Is lipopolysaccharide-induced myocardial depression mediated by cyclooxygenase products?