Project Summary Behavior is governed by contingencies that relate consequent stimuli to responses in the presence of discrete discriminative stimuli. The progression of abused drug misuse to a substance use disorder is diagnosed, in part, by behavior being misallocated towards the abused drug as the positive reinforcer and away from nondrug alternative positive reinforcers (e.g. social interaction, food, employment). Much of this scientific literature has focused on how chronic drug exposure degrades these positive reinforcing stimuli. In contrast, there are few studies that have examined how chronic drug exposure alters behaviors maintained by negative reinforcers (e.g. driving the speed limit to remove the presentation of a speeding ticket). This R21 CEBRA application proposes preclinical research in female and male rats to determine the effects of extended access fentanyl or cocaine self- administration and subsequent termination on behavior maintained by negative reinforcement. Aim #1 will establish a drug-vs-negative reinforcer choice procedure in male and female rats. Aim #2 will compare effects of extended access drug self-administration and extended access negative reinforcement on drug-vs-negative reinforcer chocie.