Naive male mice from second litters of the S9 generation, aged 55–68 days, were given a two-bottle preference version of the DID test (13). Thirty-eight mice were tested for 3 days with 2 hours access starting 3 hours after lights off. On the fourth day, access was extended for 4 hours. Half the mice were offered a single tube of 20% ethanol (v/v) each day (standard DID group). The other half of the mice were offered two identical tubes, one containing water and one containing ethanol, each day (two-bottle choice DID group). Mice within the latter groups were assigned to either left or right position of the ethanol tube, which remained the same each day, to mimic the procedure previously followed (13). A blood sample was taken from all mice at the end of the test on Day 4.
