Microdialysis and high-performance liquid chromatography with electrochemical detection were used to determine extracellular levels of dopamine following ventral tegmental morphine injections into chloral hydrate-anesthetized rats. Morphine (13.2 nanomoles in 0.5 microliter of Ringer's solution) caused 50-150% increases in nucleus accumbens dopamine and metabolites; latency for the effect was on the order of 15 min with peak effects occurring in 30-50 min. Contralateral dopamine levels were influenced only minimally. These data suggest opiate receptors in or near the ventral tegmental area as sites of the opioid action that is responsible for opioid activation of the mesolimbic dopamine system.