Mean (±SEM) number of ethanol lever responses per session (left panel), food lever responses (middle panel), and milliliters of water intake (right panel) during phase 2 for DARPP-32 KO mice (n = 7) and WT mice (n = 8). Ethanol was presented on an FR4 schedule of reinforcement. Food was presented on an FR1 schedule of reinforcement. Subjects were presented with each ethanol concentration (% v/v) for four consecutive 23 hr sessions.