Evidence suggests that two regions of the striatum contribute differential support to instrumental response selection. The dorsomedial striatum (DMS) is thought to support expectancy-mediated actions, and the dorsolateral striatum (DLS) is thought to support habits. Currently it is unclear whether these regions store task-relevant information or just coordinate the learning and retention of these solutions by other brain regions. To address this issue, we developed a two-lever concurrent variable-interval reinforcement operant conditioning task and used it to assess the trained rat's sensitivity to contingency shifts. Consistent with the view that these two regions make different contributions to actions and habits, injecting the NMDA antagonist DL-AP5 into the DMS just prior to the shift impaired the rat's performance but enhanced performance when injected into the DLS. To determine if these regions support memory content, we first trained rats on a biased concurrent schedule (Lever 1: VI 40 ′′ and Lever 2: VI 10 ′′ ). With the intent of "erasing" the memory content stored in striatum, after this training we inhibited the putative memory-maintenance protein kinase C isozyme protein kinase Mz (PKMz). Infusing zeta inhibitory peptide (ZIP) into the DLS enhanced the rat's ability to adapt to the contingency shift 2 d later, whereas injecting it into the DMS had the opposite effect. Infusing GluR2 3Y into the DMS 1 h before ZIP infusions prevented ZIP from impairing the rat's sensitivity to the contingency shift. These results support the hypothesis that the DMS stores information needed to support actions and the DLS stores information needed to support habits.It is generally believed that there are two systems that contribute to acquisition and response selection in instrumental conditioning paradigms: (1) an "action" system that allows the organism to extract relationships between its behavior, outcomes, and other environmental stimuli and represents these relationships as expectancies (Tolman 1948;Dickinson 1980;Dickinson and Balleine 1993;Rudy 2008), and (2) a "stimulus-response habit" system (S-R habit) that captures consistently occurring relationships between stimuli and responses and gradually builds stimulus-response associations that do not include representations of outcomes (Thorndike 1898). The action system provides the basis for a rapid assessment of the current environment-the contingencies and outcomes associated with behavior and the initial basis for generating an adaptive response. As responses are reliably repeated in associated stimulus contexts, the S-R habit system can automate these responses ( However, there is still a debate on the critical issue of just what these regions contribute when a new problem is encountered and solved. Do these components of the striatum coordinate the learning and retention of the solution by other regions of the relevant neural system (Pasupathy and Miller 2005), or do these regions themselves also store task-relevant information (cf. Pauli et al. 2010Pauli e...