Mice were placed in 40 × 40 cm boxes for 30 min, and their horizontal movement was recorded in six, daily sessions. (A) The time-course for the locomotion of two strains of DAT-CI mice in the first session – the C57-congenic DAT-CI mice (n = 8) habituate less efficiently than the C57:129 hybrids (n = 8). (B) The total locomotion during the first session for both the mutant (filled bars) and wild-type mice (open bars) for each of the two strains (n = 8, in all four groups). DAT-CI mice are more hyperactive than wild-type mice in both cases (*, p < 0.05 within strains; ‡, p < 0.05 between strains of DAT-CI mice). All data are means ± SEM.