Previous exposure to amphetamine sensitizes its ability to increase nucleus accumbens DA overflow in a time-dependent manner. Rats in different groups were administered five injections of amphetamine (1.0 mg/kg, i.p.) or saline, one injection every third day, and tested 2, 10–14 or 21–28 days later. On the test, in vivo microdialysis was used to estimate extracellular levels of DA in the nucleus accumbens before and after all rats were challenged with amphetamine (1.0 mg/kg, i.p.; arrows). Note that while no evidence for sensitization is evident 2 days following drug exposure, enhanced responding is greatest 21–28 days after exposure, the longest withdrawal period tested in this experiment. n/group=5–7. *, p<0.05, **, p<0.01, ***, p<0.001, significant differences between groups detected by Scheffé post hoc comparisons following ANOVA.