The potential for significant interaction between PEEP and the peripheral microcirculation is not as well appreciated as are its central circulatory effects. Therefore, we studied the effects of PEEP, 15 mm Hg, on microvascular fluid flux in the hindlimb of ten mature sheep. Changes in prefemoral lymph flow (Q̇L) and in lymph to plasma [L/P] total protein (TP) ratios were measured following the application of PEEP for 2 h, before and during hyperdynamic sepsis. Sepsis was induced by cecal ligation and perforation (CLP). Although the onset of sepsis was not associated with an increase in prefemoral Q̇L, the [L/P] ratio of iodinated 125I human serum albumin (125I-HSA) was significantly greater 72 h after CLP than during the nonseptic baseline study. Histologic examination of gastrocnemicus muscle also demonstrated an increase in protein-rich interstitial edema during the septic studies. During the 2 h of PEEP, prefemoral Q̇L increased equally (p < 0.05) in three study periods: (1) baseline nonseptic, ΔQ̇L = 1.2 ± 1.4 ml/h; (2) septic period 1, 24 to 48 h after CLP, ΔQ̇L = +1.3 ± 1.2 ml/h; and, (3) spetic period 2, 72 h after CLP, ΔQ̇L = 1.0 ± 0.6 ml/h. Calculated microvascular hydrostatic pressures also rose significantly during PEEP therapy in all three study periods. We conclude that PEEP, 15 mm Hg, increased hindlimb microvascular fluid flux and may thereby increase interstitial fluid content in tissues drained by the prefemoral lymph node. These effects of PEEP were not aggravated by hyperdynamic sepsis, despite a presumed increase in systemic microvascular permeability at this time.