Number of self-injections per day for the 3-day baseline period for cocaine (C), for consecutive days of vehicle or 100 mg/kg GHB availability, when substituted for cocaine, and for consecutive days of vehicle when substituted for GHB (extinction). Data panels shown are for the five baboons (designated BO, DS, JN, KR and XA) in which 100 mg/kg GHB maintained self-administration greater than vehicle.