Acquisition of drug-taking behavior. A) Saccharin intake. Mean ± SEM number of licks/5min of 0.15% saccharin across trials 1 – 15 for rats in the saccharin-saline, small suppressor or large suppressor group. B) Self-administration. Mean ± SEM number of saline or heroin infusions/6h across trials 1 – 15 for rats in the saccharin-saline, small suppressor, or large suppressor groups. C) Latency to 1st infusion. Mean± SEM terminal log latency to 1st heroin infusion for saccharin-saline, small suppressor, or large suppressor groups. D) Escalation. Mean ± SEM infusions during the 1st hour of self-administration in trials 1 and 15 (saccharin-saline controls: n=15, small suppressors: n=21, large suppressors: n=19). +Significant difference between large suppressors and small suppressors; *Significant difference between large suppressors and saccharin-saline controls. # Significant difference between small suppressors and saccharin-saline controls. Different letters indicate significant differences.