1. The effects of the novel mammalian tachykinin, hemokinin 1 (HEK-1), have been investigated by radioligand binding and functional in vitro and in vivo experiments. 2. Similar to SP (K(i)=0.13 nM), HEK-1 inhibited in a concentration-dependent manner and with high affinity [(3)H]-substance P (SP) binding to human NK(1) receptor (K(i)=0.175 nM) while its affinity for [(125)I]-neurokinin A (NKA) binding at human NK(2) receptor was markedly lower (K(i)=560 nM). 3. In isolated bioassays HEK-1 was a full agonist at tachykinin NK(1), NK(2) and NK(3) receptors. In the rat urinary bladder (RUB) HEK-1 was about 3 fold less potent than SP. In the rabbit pulmonary artery (RPA) HEK-1 and in the guinea-pig ileum (GPI), HEK-1 was about 500 fold less potent than NKA and NKB, respectively. 4. The responses to HEK-1 were antagonized by GR 82334 in RUB (pK(B)=5.6+/-0.07), by nepadutant in RPA (pK(B)=8.6+/-0.04) and by SR 142801 in GPI (pK(B)=9.0+/-0.2) with apparent affinities comparable to that measured against tachykinin NK(1), NK(2) and NK(3) receptor-selective agonists, respectively. 5. Intravenous HEK-1 produced dose-related decrease of blood pressure in anaesthetized guinea-pigs (ED(50)=0.1 nmol kg(-1)) and salivary secretion in anaesthetized rats (ED(50)=6 nmol kg(-1)) with potencies similar to that of SP. All these effects were blocked by the selective tachykinin NK(1) receptor antagonist, SR 140333. 6. We conclude that HEK-1 is a full agonist at tachykinin NK(1), NK(2) and NK(3) receptors, possesses a remarkable selectivity for NK(1) as compared to NK(2) or NK(3) receptors and acts in vivo experiments with potency similar to that of SP.