H2N,D-Arg,Pro,Lys,Pro,D-Phe,Gln,D-Trp,Phe,D-Trp,Leu, Leu,NH2 (L-756,867), a weak substance P antagonist, inhibited L-692,429-stimulated GH release from rat primary pituitary cells in a dose-dependent manner. At a concentration of 50 nM, L-756,867 shifted the dose-response curve of L-692,429-induced GH release to the right by about tenfold. It also impaired the ability of L-692,429 to potentiate the effect of growth hormone-releasing factor (GRF) on GH release. Substance P (1 microM) had no effect on basal or L-692,429-stimulated GH release. When tested in anesthetized rats, L-756,867 inhibited L-692,429- and growth hormone-releasing hexapeptide- (GHRP-6)-stimulated GH secretion in a dose-dependent manner. Complete inhibition was observed at an i.v. dose of 100 micrograms/kg of L-756,867. However, at the same concentration, it had no effect on GRF-induced GH secretion D-Lys3-GHRP-6, a GHRP-6 antagonist, had no effect on GHRP-6 or L-692,429-induced GH secretion even at an i.v. dose of 2 mg/kg. These results indicate that L-692,429 and GHRP-6 stimulate GH release both in vitro and in vivo via a common receptor and signaling pathway which is different from that of substance P in spite of the fact that their effects are inhibited by a weak substance P antagonist.