In vivo microdialysis measurements of extracellular dopamine (DA) were made simultaneously in nucleus accumbens (NAS) and ventral tegmental area (VTA) in anesthetized rats. Intravenous cocaine (1 mg/kg) resulted in rapid increases in DA in both NAS and VTA. The DA increase in NAS was greater in magnitude and persisted longer than that in VTA. The relevance of the observed effects to reported effects of cocaine on firing rate of mesoaccumbens DA neurons is discussed.