Interstitial fluid pressure (P_if) decreases in several experimental models of acute inflammation, enhancing edema formation. The present study was designed to determine the effect of tumor necrosis factor- (TNF-), interleukin (IL)-6, and IL-1 as well as lipopolysaccharides (LPS) on P_if in a model of gram-negative sepsis. P_if was measured in the paw skin of anesthetized rats (pentobarbital sodium, 50 mg/kg ip) using micropipettes (3-7 µm) and servo-controlled counterpressure technique. Test substances were injected intra-arterially (ia), intravenously (iv), or subdermally (sd). After intra-arterial or intravenous administration, the test substances were circulated for 1 min before circulatory arrest was induced with an intravenous injection of KCl while the rats were under pentobarbital anesthesia. Circulatory arrest was induced to avoid edema formation, which would raise interstitial fluid volume to cause a more positive P_if. Administration of 0.5 ml of LPS (5 mg/ml ia) lowered P_if significantly from control values of 0.2 ± 0.3 to 2.0 ± 0.3 mmHg (P < 0.05) within 1 h. Corresponding values for TNF- (500 ng/ml iv) were 0.4 ± 0.2 to 2.3 ± 0.1 mmHg (P < 0.05). Administration of 5 µl (5 mg/ml sd) of LPS did not affect P_if significantly (P > 0.05), but TNF-, IL-1, and IL-6 had a significant effect on P_if when given subdermally. IL-6 (50 ng/ml) caused a decrease in P_if from control values of 1.2 ± 0.3 to 2.8 ± 0.5 mmHg (P < 0.05) within 1 h. The experiments demonstrate that LPS, TNF-, IL-1, and IL-6 induce lowering of P_if when given intravenously or intra-arterially, whereas only TNF-, IL-1, and IL-6 induce lowering of P_if when given subdermally. We therefore suggest that the lowering of P_if in this experimental model of sepsis is related to the release of and a local effect in skin of TNF-, IL-1, and IL-6.