An approach for sampling the extracellular space of essentially any tissue or fluid compartment in the body. Continuous sampling can be performed for long periods with minimal perturbation to the experimental animal. Microdialysis provides a route for sampling the extracellular fluid without removing fluid, and administering compounds without adding fluid. The resulting sample is clean and amenable to direct analysis. Microdialysis was initially developed to study neurochemical processes in the brain. The success of this technique in the study of neurotransmitter release has led to the development of microdialysis techniques for general pharmacokinetic and drug distribution studies.