GR-127935 is a drug which acts as a selective antagonist at the serotonin receptors 5-HT<sub>1B</sub> and 5-HT<sub>1D</sub>. It has little effect when given by itself but blocks the antiaggressive effect of 5-HT<sub>1B</sub> agonists, and alters release of serotonin in the brain, as well as reducing drug-seeking behaviour in cocaine addicted rats.