S l ' M M A U V : In the abscncc of clirect access lo in ic r s i i t i a l lluid ( ISF) ihc concentration of thc mctabolic hormonc insul in in pcriphcral lissucs mostly had to be iisscs.scd by indirect nieasurements. Now inucroscopically pcribratcd catheters in open-fiow micTopeiTusion (OFM) allow direct sampling of ISF wi ih in thc tissues and if properly calibrated also q u u n t i t k u t i o n of ISF concentrations. In a study we placed OFM cathclers in skeletal muscle and subcutancous adiposc lissue and perfused them randonily with diffcrent insulin concentrations under hypcrinsulinacmic conditions. The mcan inlerstilial Iract ion in thc collcctcd samplcs was above 50% allowmg rcliable determination of the „no-net-flutf* concentrations by means of linear regression analysis of mtcrsiitial data. Acquired interstitial insulin levels were significantly lovver lhan those in plasma indicating rcstrictcd iranscapillary exchange of insulin in pcriphcral tissues. In respect to methodology the results dcmonstratc that open-flow microperfusion facilitates thc clarification of the pharmacokinetics of peptides and hormoncs if combined with proper strategies for ijuantification.