In the drug-free preference test, MAHDR-2 mice exhibit MA-induced conditioned place preference (CPP), whereas MALDR-2 mice do not. Shown are means ± SEM sec/min on the grid floor measured during 30-min place preference tests for animals conditioned with MA on the grid (G+) or hole (G-) floor. Measurements were taken in a standard drug-free preference test (A), in which animals (n = 9-15 per line, group and dose) were administered saline before the test. Differences in pharmacological effects of MA during the same place preference test (B) are also presented as, percent (%) time on MA-paired floor. ***p<.001 for the main effect of conditioning group (A), or for the main effect of line (B). MAHDR-2, MA high drinking replicate 2; MALDR-2, MA low drinking replicate 2.