Levels of dopamine, both basal and after binge-pattern cocaine administration, were measured in the caudate putamen and nucleus accumbens of C57BL/6J and 129/J mice by in vivo microdialysis. Six-week old males were surgically implanted with a CMA guide cannula into the caudate putamen or nucleus accumbens. After 4 days recovery, dialysis probes were lowered into the caudate putamen or the nucleus accumbens and mice were placed in individual microdialysis chambers. The next morning experiments were carried out on freely moving animals. Experimental animals received 1-day binge cocaine administration (15 mg/kgx3, i.p. at hourly intervals) while control animals received saline in the same pattern. Dialysates were collected every 20 min and dopamine content was determined by HPLC with electrochemical detection. Basal levels of dopamine in the dialysate of the caudate putamen were 4.2+/-0.2 nM in C57BL/6J mice and 5.0+/-0.3 nM in 129/J mice. In the nucleus accumbens, basal levels of dopamine were 0.65+/-0.04 nM in the C57BL/6J mice and 0.75+/-0.03 nM in 129/J mice, with no significant differences between strains in either region. Binge cocaine administration significantly increased mean dopamine levels in the caudate putamen in the C57BL/6J mice (with a 3-h mean of 6.80 nM) and in the 129J mice (9.94 nM). In this region, 129/J mice had significantly higher levels of cocaine-induced dopamine than did C57BL/6J mice. In the nucleus accumbens, administration of cocaine also significantly increased dopamine levels in both strains (1.32 nM in C57BL/6J and 1.43 nM in 129/J), but with no difference between strains.