Locomotor activity levels in methamphetamine (MA) drinking line mice during the 6 conditioning trials for (A) saline and (B-D) MA. Locomotor activity was measured as photocell beam breaks during the 15-min conditioning sessions. Shown are means ± SEM total beam breaks. Conditioning sessions were alternated between saline and MA in a counterbalanced order across animals. MA dose groups were independent, so there were separate saline conditioning trials for each group. Since there were no significant MA dose group effects on activity levels during the saline trials, these data are presented collapsed on MA dose group. ‡p<.05 significant mean difference between trial 1 and 6, indicating locomotor sensitization (data were collapsed on line for these comparisons, because there were no significant line differences). #p<.05 significant mean differences between trial 1 and all subsequent trials for MAHDR-2 mice. *p<.05 for the comparison of MAHDR-2 and MALDR-2 line means. n = 20-25 per line and dose group. MAHDR-2, MA high drinking replicate 2; MALDR-2, MA low drinking replicate 2.