Infectious reactions are known to comprise changes in vasopermeability and inflammatory mediators. We used peritonitis that complicated continuous ambulatory peritoneal dialysis (CAPD) as an in vivo inflammation model to study the time courses of peritoneal permeability characteristics and mediators in dialysate. Sixteen episodes of peritonitis were prospectively followed on eight consecutive days from the onset of the infection and once after recovery (control). Dialysate night dwells were examined for marker proteins of peritoneal permeability, cytokines (tumor necrosis factor-alpha [TNF-alpha], interleukin-6 [IL-6] and prostaglandins (PGE2, 6-keto-PGF1 alpha, and thromboxane B2 [TxB2]). The clearance of beta 2-microglobulin was used as indicator of the effective peritoneal surface area. The intrinsic permeability was characterized functionally by the peritoneal restriction coefficient. All protein clearances were increased during the acute phase of peritonitis and subsequently decreased to control. Likewise, the intrinsic peritoneal permeability was elevated, as demonstrated by a decrease of the peritoneal restriction coefficient. Peritoneal appearance rates of TNF-alpha, IL-6, and prostaglandins were also increased during the acute phase. Peak values were reached on day 1. The largest increase was observed for IL-6 (median 854-fold), followed by TNF-alpha (35-fold). The vasodilating PGE2 and 6-keto-PGF1 alpha were increased 12-fold and the vasoconstricting TxB2 was increased threefold. Evidence was obtained for local intraperitoneal synthesis of IL-6 and prostaglandins. TNF-alpha production appeared to be present only during the early acute inflammatory response. Analysis of variance for repeated measurements revealed that changes in the clearance of beta 2-microglobulin were related to those in IL-6 and marginally also to TNF-alpha in dialysate. Changes in the peritoneal restriction coefficient were also related to IL-6, but were more closely related to alterations in dialysate PGE2. These findings suggest that TNF-alpha, IL-6, and PGE2 are involved in the changes in permeability characteristics during CAPD-related peritonitis.