Fluctuations in extracellular dopamine and DOPAC levels in nucleus accumbens septi (NAS) were monitored in 1-min microdialysis samples taken from rats engaged in intravenous cocaine self-administration. For four rats the dose per injection was fixed at 2.0 mg/kg; for four others the dose per injection was varied irregularly, from one response to the next, between three levels (0.5, 1.0 and 2.0 mg/kg). Regardless of the dosing regimen, extracellular dopamine levels were tonically elevated by 200-800% within the cocaine self-administration periods, fluctuating phasically within this range between responses. In the fixed dose condition, the phasic increases following each injection (and the phasic decreases preceding them) averaged approximately 50% of the mean tonic elevation. Phasic fluctuations in dopamine levels remained time-locked to lever-presses even when response rate was irregular, because of the variable dose condition. In the variable dose condition greater increases in dopamine and longer inter-response times followed injections of the higher doses; dopamine fluctuations were consistent with the multiple-infusion pharmacokinetics of cocaine. DOPAC levels showed a slow tonic depression during cocaine self-administration, but individual injections were not associated with discernible phasic fluctuations of DOPAC. These data are consistent with the hypothesis that falling dopamine levels trigger successive responses in the intravenous cocaine self-administration paradigm, but inconsistent with the notion that extracellular dopamine levels are depleted at the times within sessions when the animal initiates drug-seeking responses.