This paper presents evidence from microdialysis experiments that three different behavioural outcomes of depletion of striatal dopamine (DA) by either 6-hydroxydopamine or methamphetamine (sparing of function, recovery of function, and loss of function) may be related to differences in the ability of residual DA neurons to maintain extracellular concentrations of DA. It is shown that when up to 80% of the mesostriatal DA system is destroyed, the remaining DA terminals maintain normal extracellular dopamine concentrations. Following a lesion in the 80-95% range, most animals maintain a relatively normal extracellular concentration of DA, but the ability to respond to increased demand is reduced in some animals. When over 95% of the normal DA input to the striatum is destroyed, there is a sharp drop in the extracellular concentration of DA and a nearly complete loss in the ability to increase DA release upon demand. Thus, this new method of sampling extracellular DA and its metabolites in freely moving animals provides insights into the behavioural capacities of animals and into the neural mechanisms that underlie recovery of function following brain damage.