Active lever presses (A) and inactive lever presses (C) at saline, 0.1, 0.3, 1, 3, or 10 μg/kg/infusion remifentanil by rats trained with 1 μg/kg remifentanil. Active lever presses (B) and inactive lever presses (D) by rats trained with 3 μg/kg remifentanil. Enr: gray triangles (n = 7 for 1 μg/kg; n = 5 for 3 μg/kg), Std: black squares (n = 7 for 1 μg/kg; n = 5 for 3 μg/kg), Iso: white circles (n = 7 for 1 μg/kg; n = 5 for 3 μg/kg). Note the difference in scales between the upper and lower panels.