Estrogen has been shown to enhance the effects of antipsychotics in humans. To investigate the mechanisms of how this may occur, the current study examined estradiol's effects on dopaminergic transmission and behavior in amphetamine-sensitized and non-sensitized female rats. Sixty-four ovariectomized female Sprague–Dawley rats were used for this study. Half of the rats were sensitized to four once-daily injections of 1 mg/kg amphetamine and the other half served as controls. Rats received chronic administration of either low-dose haloperidol (0.25 mg/kg/day) or saline vehicle via osmotic minipumps implanted subcutaneously. The groups were further subdivided with respect to estradiol treatment: low chronic estrogen (subcutaneous estradiol implant, 0.36 mg/pellet: 90-day release, plus an additional oil vehicle injection every second day) and high pulsatile estrogen (subcutaneous estradiol implant plus an additional 10 μg/kg estradiol injection every second day). Motor activity was assessed at day 2 and day 12 during haloperidol treatment, while nucleus accumbens dopamine availability was assessed via microdialysis 10 days into antipsychotic treatment. Haloperidol treatment along with high, but not low, estradiol replacement was effective in reducing amphetamine-induced locomotor activity in sensitized rats. High estradiol treatment also augmented the effects of chronic haloperidol in reducing dopaminergic release in sensitized rats. These data suggest that estradiol levels affect both the behavioral and the dopamine responses to chronic antipsychotic treatment.