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Maintenance of WIN55,212-2 (WIN) self-administration in Long Evans (LE; left panel) and Lister Hooded (LH; right panel) female and male rats. Each bar represents the number of active lever-presses (mean±s.e.mean) during each daily 2-h session, over the 5 days of this phase. Each group included seven animals. **P<0.01 vs corresponding values for males.
