The goal of this project is to develop microdialysis sampling methods for the study of pharmacologically important systems.  Microdialysis sampling is accomplished by implanting a short length of dialysis fiber at the site of interest.  This fiber is slowly perfused with a sampling solution such that small molecular weight compounds in the sample diffuse into the fiber and are swept away to be collected for analysis.  While originally developed for intracerebral sampling of neurochemical systems, we have demonstrated the utility of microdialysis sampling of the blood stream and peripheral tissue for pharmacokinetic investigations.  A new microdialysis probe design will be used to sample the portal vein to determine the bioavailability of orally administered drugs and to determine the effect of first-pass metabolism.  Methods will also be developed for sampling the vitreous humor of the eye as an important pharmacological site not well represented by blood concentrations.  Sampling this site will also require the development of a new microdialysis probe design.  Development of dermal sampling methods will be continued.  Enhanced microanalytical techniques are needed to support the development of microdialysis because of the small volume, low concentration samples resulting from this technique.  Two on-line systems will be developed to achieve near real-time analysis during a microdialysis experiment.  In the first system, microdialysis sampling will be coupled to fast microbore liquid chromatography.  In the second system, microdialysis sampling will be coupled on-line to capillary electrophoresis.  Fundamental aspects of mass transport in the microdialysis system will also be investigated.  An in vitro flow apparatus has been designed to simulate the blood stream to study microdialysis under hydrodynamic conditions.  Both methods for calibrating the probe for in vivo use and parameters for optimal sampling conditions will be investigated.  Calibration techniques based on the delivery mode of microdialysis will be studied for applicability to calibration for sampling tissues.