The importance of selective regulators of thyroid functions is dictated by a key role the hypothalamic-pituitary-thyroid axis plays in the control of biochemical and physiological processes and by the fact that only a limited number of drugs with thyrotropic potency is available today. We synthesized peptide corresponding to 612-627 region of third intracellular loop of rat thyroid-stimulating hormone (TSH) receptor and its palmitoylated analogue 612–627-K(Pal)A and studied their influence on adenylyl cyclase (AC) signaling system in the thyroid and the extrathyroidal tissues. Peptide 612–627-K(Pal)A at micromolar concentrations stimulated basal AC activity and GppNHp binding of G_s and G_q proteins and inhibited TSH-stimulating AC activity in the thyroidal membranes but had little influence on AC system in synaptosomal, myocardial and testicular membranes, which indicates the receptor and tissue specificity of 612–627-K(Pal)A. In the thyroid 612–627-K(Pal)A also decreased PACAP-38- and human chorionic gonadotropin-stimulated AC activity, which is most likely to be due to inhibiting of TSH-mimicking effect of these hormones on AC. Unmodified peptide 612–627-KA was effective to a much lesser extent, indicating the importance of hydrophobic moiety for 612–627-K(Pal)A activity. The obtained data suggest the possibility to develop thyroid regulators and modulators on the basis of TSH receptor-derived peptides.