Fig. 3.

Percent choice in four monkeys for three mixture ratios of cocaine/remifentanil predicted to be equivalent in reinforcing effect to 100 μg/kg/inj of cocaine if the drugs were additive. The alternatives to the mixtures were cocaine at 100 μg/kg/inj (Coc 100; black bars), 300 μg/kg/inj (Coc 300; dark gray bars) or 560 μg/kg/inj (Coc 560; light gray bars), and remifentanil at 1.0 μg/kg/inj (Remi 1.0; horizontally hatched bars) or 1.7 μg/kg/inj (Remi 1.7; diagonally hatched bars). Missing bars indicate conditions that were not run, not zero percent choice