Rats were treated twice daily either with saline or d-amphetamine (5 mg/kg) for 5 days. When challenged approximately 15 days later with an injection of 0.5 mg/kg amphetamine, the chronic amphetamine animals showed (1) an augmented release of dopamine in the caudate nucleus in vivo and (2) an increase in stereotyped behavior compared to the chronic saline animals. These results suggest that an increase in dopamine release from the caudate may contribute to amphetamine-induced behavioral sensitization.