Docetaxel and prostaglandin E1 (PGE1) increase transcapillary albumin extravasation and reduce interstitial fluid pressure in the skin. In this study the microdialysate concentration (Cm) of 125I-labeled human serum albumin (125I-HSA) and different-sized endogenous plasma proteins (EPP) was compared to evaluate changes in transcapillary extravasation of plasma proteins. 125I-HSA was also used to estimate changes in the specific activity of albumin. Extravasation of 125I-HSA and EPP from plasma to interstitium in the rat skin was compared during continuous administration of docetaxel and PGE1 by using microdialysis in anesthetized rats. Also, 20 ml of Ringer solution (RS) were injected intravenously during 10 min in a separate group. Two hollow plasmapheresis fibers (3 cm, cut off 3,000 kDa), one acting as control, were placed subcutaneously on the back skin and perfused with RS (5 microl/min, 140 min, collected every 10 min). The size of the different EPP was estimated to be 73, 65, 56, 47, and 39 A, separated by a size-exclusion high-performance liquid chromatography column and quantified by UV detection (280 nm). Docetaxel (0.5 mg/ml, n = 5) increased Cm of 125I-HSA and EPP of sizes 73, 65, 56, and 39 A significantly (P < 0.05) compared with control. PGE1 (20 microg/ml, n = 6) increased Cm of 125I-HSA significantly (P < 0.05) but none of the different-sized EPP was increased compared with control. Intravenous RS (20 ml, n = 6) increased Cm of 125I-HSA and increased all the different-sized EPP significantly (P < 0.05) compared with control. Although the microdialysis method is able to monitor qualitative changes in capillary permeability, a quantitative determination of the capillary reflection coefficient or permeability-surface area product was not possible, because steady state between plasma and dialysate was not achieved during the measurement period. The different pattern of extravasation of EPP and 125I-HSA after docetaxel, PGE1, and RS indicates increased interstitial transport rate and/or increased capillary permeability after docetaxel and RS, whereas PGE1 seems to increase transcapillary fluid flux without altering the permeability.