Fig 2.

Choice between a dose of remifentanil and a dose of cocaine 20.5 hr after daily administration of 3.2 mg/kg/inj morphine in 4 rhesus monkeys. Percent choice of remifentanil is shown on the left ordinate and rates of responding maintained by the doses of remifentanil and cocaine are shown on the right ordinate. The initial data point and the horizontal dashed line represent premorphine choice; the error bars are standard deviations. Daily sessions are shown on the abscissa. Note that the dose of remifentanil is 0.03 µg/kg/inj for Monkey 3506 (bottom) and 0.3 µg/kg/inj for the other monkeys.