The subjects were 3 adult rhesus monkeys (Macaca mulatta), 2 male (Monkeys 3572 and 3603) and 1 female (Monkey 3600), with a history of drug self-administration including remifentanil. Monkeys lived in the experimental chambers and were fed 10 to 15 Purina monkey chow biscuits twice daily (at least 1.5 hr before experimental sessions) to maintain their body weights. Daily fresh fruit and other treats supplemented this diet. Water was continuously available. In accordance with institutional animal care and use requirements, environmental enrichment toys also were provided on a regular rotating basis.

Apparatus
Monkeys were permanently housed in stainless steel cages (83.3-cm long by 76.2-cm wide by 91.4-cm deep). The front, top, and bottom of the cage were made of barred stainless steel, and a pan was located below the floor to collect waste. Located on the wall to the left of the barred front door was an intelligence panel 20 cm in length and 15.4 cm in height, approximately 10 cm from the front and 19 cm from the bottom of the cage. Across the top of the stimulus panel, 1.5 cm apart, were three circular openings, 2.5 cm in diameter, covered with translucent plastic and capable of being illuminated from behind with 5-W colored bulbs. The two side lights could be illuminated red and the center light green. Centered below the right and left stimulus lights were response levers (Model 121-07, BRS-LVE) capable of being operated by 10 to 15 g (0.10–0.15 N) of force. A 0.3-cm thick stainless steel divider, centered between the response levers and below the stimulus lights, extended 8 cm into the chamber. Experimental control was provided by an IBM PS/2 computer located in an adjoining room and programmed with Med-PC (Med-Associates, Georgia, VT) software.
 
Each monkey wore a Teflon mesh jacket (Lomir, Quebec, Canada) connected to a flexible stainless-steel spring tether attached to the rear of the cage. Monkeys had been implanted previously with indwelling intravenous catheters in an internal or external jugular, or femoral vein, under ketamine (10 mg/kg, IM) and xylazine (2 mg/kg, IM) anesthesia. Catheters were run subcutaneously from the site of implantation to an exit site in the middle of the back. Tubing was then fed through the steel spring tether and passed to the outside rear of the cage where it was connected to a stock solution of remifentanil (either 0.4 µg/kg/ml or 1.2 µg/kg/ml) and additional infusion lines that passed through the rollers of two infusion pumps. Different doses were arranged by manipulating the speed of the two pumps. Operation of one pump delivered 0.15 ml solution per s. Operation of the other delivered 0.05 ml solution per s. Injections were always 5 s in duration. When the stock solution was 0.4 µg/kg/ml remifentanil, operation of the faster pump resulted in a delivery of 0.3 µg/kg/inj (0.4 µg/kg/ml × 0.15 ml/s × 5 s) and operation of the slower pump delivered 0.1 µg/kg/inj. When the stock solution was prepared at 1.2 µg/kg/ml, doses of 0.9 and 0.3 µg/kg/inj were arranged.