Mean (+SEM) reinforced and non-reinforced responding was plotted for days 5–9 for the three reinforcement ratios. Animals received (±) TCP (3 mg/kg, i.p.) 1 hr (A) or 20 hr (B) before nicotine (7.5 μg/kg/inj, i.v.) self-administration. The 1-hr pretreatment group maintained self-administration at all ratios, but the 20-hr group did not (*p<0.05, ***p<0.001 vs. Non-Reinforced; ++p<0.01 vs. previous day, n=21). Tests of (+)TCP (C) and (−)TCP (D) at (1.5 mg/kg, i.p.) 1 hr prior to nicotine self-administration showed that the (+)TCP-pretreated animals maintained self-administration at all ratios, but the (−)TCP-pretreated rats did not (*p<0.05, **p<0.01 vs. Non-Reinforced, n=12/group).