The invention provides an improved microdialysis method for determining the free concentration of an agent or drug (defined as the concentration of agent that is dissolved and free to diffuse, and not undissolved or precipitated, complexed, bound, included in micelles or microemulsions, etc.).In addition. the invention provides such a method under conditions in which the properties of the microdialysis probe may change.Further, the invention provides a method for determining the permeability of a diffusible agent through the probe wall.The invention also provides a method for determining the rate of change of concentration of an agent in a medium.
