Morphine sulfate was provided by the National Institute on Drug Abuse Drug Supply Program (Bethesda, MD) and was dissolved in sterile water. 2-[(1R,2R,5R)-5-hydroxy-2-(3-hydroxypropyl) cyclohexyl]-5-(2-methyloctan-2-yl)phenol (CP55940; Sigma-Aldrich, St. Louis, MO) was dissolved in pure ethanol (2 ml ethanol per bottle containing 10 mg CP55940) then diluted with a solution of 1:1:18 ethanol:emulphor:saline. Drugs were administered subcutaneously in a volume of 0.2 – 0.8 ml. Pretreatment times were 15 min for morphine and 60 min for CP55940; behavioral effects of morphine and CP55940 are apparent with 15 and 60 min, respectively, and last at least 90 min in various operant-conditioning tasks, including delay discounting (e.g., Maguire et al., 2012 (link), 2013 (link), 2016 (link)). All doses were expressed as the weight of the forms noted above.