Whether serotonin-1B (5-HT1B) receptor activation enhances or diminishes the reinforcing properties of psychostimulants remains unclear. We have previously shown that increased expression of 5-HT1B receptors in nucleus accumbens (NAcc) shell neurons sensitized rats to the locomotor-stimulating and rewarding properties of cocaine. In this study we further examined the contribution of 5-HT1B receptors on the effect of cocaine under conditions intended to selectively influence either conditioned place preference or avoidance (CPP or CPA, respectively). Viral-mediated gene transfer techniques were used to overexpress 5-HT1B receptors in medial NAcc shell medium spiny neurons projecting to the ventral tegmental area. Animals were then conditioned to associate place cues with the effects of either a low (5 mg/kg) or moderately high (20 mg/kg) dosage of cocaine immediately or 45 min after intraperitoneal cocaine administration. Animals with increased 5-HT1B expression showed cocaine-induced CPP immediately after administration of the low 5 mg/kg dose of cocaine, but a CPA 45 min after administration of the high 20 mg/kg dose. Control animals showed no preference at the 5 mg/kg dose and a significant preference at 20 mg/kg. Given this, we believe that increased 5-HT1B receptor activation in NAcc shell projection neurons intensifies both the rewarding and negative properties of cocaine use.