Abstract: The present invention relates to a new class of G-protein-coupled receptor antagonists which bind to the intracellular molecular interface between the receptor and the G-protein, thus hampering signal transduction. The present invention describes peptide sequences derived from the prostaglandin receptor F.sub.2.alpha. and the G-protein, G.alpha.q protein, produced by molecular biology techniques or chemical synthesis, as selective inhibitors of signal transduction involved in the stimulation of this receptor. Such peptides or molecules derived from their primary, secondary and tertiary structures may be used as effective tocolytics for the prevention of premature labor or be utilized for the treatment of dysmenorrhea.