Table 1.

Number of reinforcers (± SEM) earned in a five-minute response component for each dose of agonist tested.

	Reinforcement density
Oxycodone	0.032 mg/kg	0.1 mg/kg	0.32 mg/kg	0.56mg/kg
	9.25 ± 0.25	9.00 ± 0.00	8.67 ± 0.33	0.50 ± 0.50
Heroin	0.01 mg/kg	0.032 mg/kg	0.1 mg/kg	0.32 mg/kg
	9.00 ± 0.00	9.00 ± 0.00	8.00 ± 0.71	3.75 ± 2.25
Methadone	0.032 mg/kg	0.1 mg/kg	0.32 mg/kg	1.0 mg/kg
	9.00 ± 0.00	8.25 ± 0.75	8.00 ± 0.41	1.75 ± 1.03
Buprenorphine	0.0032 mg/kg	0.01 mg/kg	0.032 mg/kg	0.1 mg/kg
	9.00 ± 0.00	8.25 ± 0.48	6.00 ± 2.12	1.00 ± 1.00
Butorphanol	0.01 mg/kg	0.032 mg/kg	0.1 mg/kg	0.32 mg/kg
	8.00 ± 0.58	8.00 ± 0.58	6.00 ± 0.71	2.25 ± 1.03
Nalbuphine	0.1 mg/kg	0.32 mg/kg	1.0 mg/kg	3.2 mg/kg
	8.00 ± 1.00	8.75 ± 0.48	8.00 ± 0.58	7.25 ± 1.03