The 5-HT1A/B agonist RU 24969 induces hyperactivity in rodents and also shows antidepressant-like effects in some animal models of depression. We have examined the effects of selective antagonists at 5-HT1A and 5-HT1B/D receptors (WAY 100135 and GR 127935, respectively) on both the hyperlocomotor and anti-immobility effects of RU 24969. While a high dose of WAY 100135 (10 mg/kg) had no effect in either paradigm, GR 127935 attenuated the behavioural effects of RU 24969 in both. WAY 100135 was also without effect on the antidepressant effect of paroxetine, while GR 127935 blocked the effects of paroxetine (1 mg/kg) and imipramine (10 mg/kg). Furthermore, while coadministration of paroxetine or imipramine enhanced the effects of RU 24969 in the mouse tail suspension test, imipramine had no effect on the locomotor activating effects of the 5-HT1B agonist, suggesting different neural substrates may underly the effects in the different tests. These studies indicate a role for the 5-HT1B/D receptor in the mediation of the effects of antidepressant treatment.