A microdialysis method is described which allows for the quantitative determination of extracellular analyte concentration under transient conditions. The method provides the extracellular concentration and the in vivo probe recovery as a function of time. The technique is based on the method for steady-state conditions but differs in the use of a between-group rather than a within-group design. Following cocaine and amphetamine administration, a significantly greater increase in extracellular DA was found than was estimated from the dialysate using conventional microdialysis methods. The discrepancy is due to microdialysis probe recovery decreasing concurrently with the increase observed in extracellular DA following cocaine and amphetamine injections. The method estimates the extracellular concentration independently of any changes in recovery.