Tachykinin receptors in rat gastric fundus were characterized using radioligand binding, functional and autoradiographic techniques. In crude homogenates of fundus, the specific binding of 125I-iodohistidyl-neurokinin A (INKA), 125I-Bolton-Hunter eledoisin (BHELE) and 125I-Bolton-Hunter [Sar9,Met(O2)11]-SP (BHSar-SP) was reversible and saturable. INKA and, in particular, BHSar-SP showed high affinity binding (Kds, 2.2 and 0.6 nM, respectively), with lower affinity for BHELE (Kd, 17 nM). The binding capacity was higher for INKA and BHELE than for BHSar-SP. The superior potency of neurokinin (NK)-2-preferring agonists (neuropeptide gamma > or = [Lys5,MeLeu9,Nle10]-NKA(4-10) > or = neuropeptide K > neurokinin A [NKA] > [Sar9,Met(O2)11]-SP > senktide) and antagonists (SR 48,968 > GR 94,800 > MDL 29,913 > L-659,877 > MEN 10,207) as competitors for INKA and BHELE binding suggests interaction at mainly NK-2 sites. Additional competition studies showed that BHSar-SP was binding to NK-1 sites. Autoradiographic studies revealed very dense INKA and BHELE specific binding over the circular muscle and muscularis mucosae, while BHSar-SP binding was observed only to the circular muscle. The weak specific binding for 125I-Bolton-Hunter scyliorhinin II localized to the muscularis mucosae may indicate NK-3 sites. This was consistent with functional studies showing concentration-dependent contractions of fundus strips by NK-2-preferring tachykinin agonists (potency, pD2s, 7.1 to 8.1) and [Sar9, Met(O2)11]-SP (pD2, 7.1). The NK-2 selective antagonist MDL 29,913 inhibited INKA binding (Kd, 14 nM) with more than tenfold greater affinity than did MEN 10,207. The antagonism by MDL 29,913 was noncompetitive, with a nonparallel rightward shift of the concentration-response curves to the agonists neuropeptide gamma, neuropeptide K, NKA and [Lys5,MeLeu9,Nle10]-NKA(4-10) (dose ratios at 400 nM MDL 29,913 were 230, 62, 40 and 23, respectively). These data indicate that classic NK-2 receptors predominate in the rat fundus and that NK-1 and perhaps NK-3 receptors also exist.