The effects of tetrodotoxin (TTX) and K+ perfusion on extracellular DA concentration (DAext) and extraction fraction (also known as relative recovery or Ed) were studied using a method for quantitative microdialysis under transient conditions. TTX administered through the microdialysis probe decreased DAext with no effect on Ed. Perfusion with high K+ increased DAext, with no initial effect on Ed. However, a decrease in Ed was observed under prolonged depolarization. These data provide further evidence that Ed may change over the course of drug administration. This method of quantitative microdialysis is a reliable means for measuring changes in DAext independently of possible changes in microdialysis probe efficiency.