Mean (±1 SEM) active lever responding for rats in the NIC-Only, NIC + VS, and 2-Lever groups with access to 30 (a), 60 (b), or 90 μg/kg (c) nicotine infusions. For the 2-Lever group, responding on the infusion lever (■) is depicted separately from responding on the VS lever (□). The VS-Only group (0 μg/kg/infusion) is depicted in each panel for comparative purposes. When nicotine and VS were contingent upon a single response (NIC + VS groups) higher unit doses decreased response rate. In contrast, when nicotine and VS were contingent upon separate responses (2-Lever groups), higher unit doses still potently increased responding for VS.