Mice were placed into a 25 × 25 cm open-field apparatus and allowed to habituate for 45 min. (A) Wild-type mice from both strains were injected with amphetamine at the doses indicated (0, 2.5, 5, and 20; in mg/kg), and returned to the apparatus for another 45 min. The locomotion over the 30 minutes before injection (open bars) and after injection (filled bars) is plotted. Both strains of wild-type mice have locomotor stimulation, but never suppression, from amphetamine (*, p < 0.05; relative to pre-injection locomotion). (B) DAT-CI mice from both strains were also injected with amphetamine at the same doses, plus 10 mg/kg, and measured in the same way. Both strains of DAT-CI mice have suppression of locomotion from 2.5 mg/kg amphetamine, but only the C57:129 hybrid DAT-CI mice have locomotor stimulation from 20 mg/kg. All statistics are comparisons of post-injection locomotion relative to the corresponding pre-injection score. All data are means ± SEM.