Effects of injection speed on cocaine’s acute reinforcing effects in animals showing or not showing sensitization. Following testing on a PR schedule for 14 days at a particular injection speed (see Fig. 2), animals were tested with each injection duration during PR and FR sessions. (A) Breakpoints maintained in animals with a history of 5-s injection durations (i.e. sensitized animals) were higher than those in the 25- and 50-s groups, regardless of the current injection speed conditions (F2,55 = 12.84, P < 0.05). Data are represented as mean (± SEM) breakpoints. (B) Rate of intake (injections per hour) increased as the speed of injection decreased across all groups. There were no differences among these curves across groups (speed: F2,34 = 3.83, P < 0.05; group: F2,16 = 0.11, P > 0.05).