Recent studies have shown that antagonists of serotonin (5-HT) 1B receptors attenuate cocaine-induced locomotor hyperactivity, whereas agonists enhance reinforcing and discriminative stimulus effects of the psychostimulant. The present study was designed to determine how 5-HT 1B receptor ligands affected the development or the expression phase of sensitization to the cocaine-induced locomotor response in rats. In Experiment 1, rats were treated repeatedly (for 5 days) with cocaine (10 mg/kg) in combination with either saline, GR 127935 (5-HT 1B antagonist), CP 94,253 (5-HT 1B agonist) or GR 127935 + CP 94,253. On day 10, they received a challenge dose of cocaine (10 mg/kg). In Experiment 2, animals received either saline or cocaine (10 mg/kg) for 5 days, and were then challenged with cocaine (10 mg/kg) in combination with saline, GR 127935, CP 94,253 or GR 127935 + CP 94,253, on day 10. In Experiment 3, rats received either saline, cocaine or CP 94,253 for 5 days; on day 10 they received challenge doses of CP 94,253 or cocaine. In rats treated repeatedly with cocaine, the locomotor hyperactivity induced by a challenge dose of the psychostimulant was about twice as high as that observed after its first administration. The effect evoked by cocaine challenge was further increased in animals treated repeatedly with CP 94,253 + cocaine, but not with GR 127935 + CP 94,253 + cocaine. No difference was observed in the response to cocaine challenge in rats treated repeatedly with cocaine or GR 127935 + cocaine (Experiment 1). In animals treated repeatedly with the psychostimulant, the behavioral response to a challenge dose of cocaine was dose-dependently increased when that drug was combined with CP 94,253, but not with GR 127935 + CP 94,253. No difference was observed in the locomotor response of rats challenged with cocaine or GR 127935 + cocaine (Experiment 2). When rats were treated repeatedly with cocaine, a challenge dose of CP 94,253 produced an about threefold increase in the locomotor effect compared to the animals treated likewise with saline (Experiment 3). Our results indicate that 5-HT 1B receptors are involved in neither the development nor the expression of sensitization to cocaine-induced locomotor hyperactivity. On the other hand, they also show that pharmacological activation of 5-HT 1B receptors enhances both phases of this phenomenon, and that repeated administration of cocaine leads to an increased functional reactivity of these receptors.