A method is described for monitoring extracellular levels of striatal dopamine in the rat during behavior. The extracellular fluid is sampled using a microdialysis probe modified for use in behaving animals. Dopamine concentration in the perfusate is determined every 5 min using an automated smallbore chromatographic system with electrochemical detection. The system is capable of detecting behaviorally related changes of 5 nM, in extracellular dopamine.