Table 1.

Fentanyl intravenous self-administration. Mean number of infusions self-administered at fixed ratio (FR) 1, FR2, FR5, and progressive ratio (PR) schedules of reinforcement at 0, 1.25, and 2.5 μg/kg/infusion.

Fentanyl Dose	Number of Animals	Results
0 μg/kg/infusion	15	Mean infusions: FR1 D4–5 Reinf: 11.80 FR1 D4–5 NonReinf: 10.60 FR2 D6–7 Reinf: 6.26 FR2 D6–7 NonReinf: 5.76 FR5 D8–9 Reinf: 4.0 FR5 D8–9 NonReinf: 4.26 PR: 2.56
1.25 μg/kg/infusion	16	Mean infusions: FR1 D4–5 Reinf: 20.90 FR1 D4–5 NonReinf: 14.65 FR2 D6–7 Reinf: 20.43 FR2 D6–7 NonReinf: 12.53 FR5 D8–9 Reinf: 13.25 FR5 D8–9 NonReinf: 9.53 PR: 8.81
2.5 μg/kg/infusion	10	Mean infusions: FR1 D4–5 Reinf: 16.45 FR1 D4–5 NonReinf: 12.40 FR2 D6–7 Reinf: 13.80 FR2 D6–7 NonReinf: 8.85 FR5 D8–9 Reinf: 10.70 FR5 D8–9 NonReinf: 8.15 PR: 6.90