It has been demonstrated recently that antagonists of the tachykinin NK1 receptor, specifically CP-99,994 and GR203040, possess anti-emetic activity in a range of species. To optimise this activity, a series of analogues based around the structure of GR203040 have been synthesised and their affinity at the human tachykinin NK1 receptor determined. In addition, the potency of these analogues to inhibit emesis induced in the ferret by whole-body X-irradiation has been examined. A range of substitution at the C-1 position of the tetrazole moiety in GR203040 were explored in vitro and in vivo. The trifluoromethyl compound, GR205171, was the most potent antagonist with regard to the ability to inhibit emesis induced by X-irradiation. This compound was demonstrated to have a broad spectrum of anti-emetic activity, inhibiting emesis in the ferret induced by cisplatin, cyclophosphamide, morphine, ipecacuanha and copper sulphate. Furthermore, emesis was also inhibited in the house-musk shrew, Suncus murinus, when induced by either motion or cisplatin, and in the dog when induced by ipecacuanha. GR205171 has the most potent anti-emetic activity of any tachykinin NK1 receptor antagonist described to date. The compound is orally active in the ferret and dog, long-lasting, and warrants further investigation as a potential broad-spectrum anti-emetic agent.