Biosensors hold promise for continuously monitoring metabolic diseases such as diabetes. Yet, numerous clinically relevant analytes are not amenable to direct in vivo biosensing. For these analytes, a better approach is to develop in vivo sampling methods. Over the past few years, our group has developed various microdialysis sampling approaches for in vivo collection and detection of important peptides and proteins. Microdialysis sampling is a technique that involves passing a perfusion fluid at low flow rates (0.5 µl/min to 2 µl/min) through a hollow-fiber semi-permeable membrane with dimensions of 200 to 500 µm o.d. and 1 to 30 cm in length. Microdialysis sampling probes have tremendous potential to be applied in the clinic since they can be used to recover and deliver agents directly at a tissue implant site. A significant research effort in our group has focused on creating affinity-trapping methods to trap cytokines with either antibodies or heparin-conjugates attached to beads that can be passed through microdialysis probes. We have also developed LC/MS approaches to determine the presence of matrix metalloproteinases external to microdialysis sampling probes. Finally, we have recently worked on affinity approaches to improve microdialysis sampling collection of neuropeptides related to addiction. NIH EB 001441 and DA 020577 support this research.