G-Protein antagonist peptide TFA is a truncated substance P-related peptide, competes with receptor for G protein binding. G-Protein antagonist peptide TFA inhibits the activation of Gi or Go by M2 muscarinic cholinergic receptor (M2 mAChR) or of Gs by beta-adrenergic receptor in the reconstituted phospholipid vesicles, assayed by receptor-promoted GTP hydrolysis.