Irrespective of their nature cationic peptides interact with components of the hormonal signaling systems, exerting influence on their activity and on signal transduction via these systems. We compared the influence of linear, star-like and dendrimer cationic peptides, non-homologous to G protein-coupled receptors (GPCRs), and cationic peptides, the derivatives of C-terminal region of the third intracellular loop (C-ICL3) of GPCRs, on the functional activity of adenylyl cyclase (AC) signaling system and the transduction of hormonal signal via this system. It was found that non-GPCR and GPCR-derived peptides both are activators of G proteins. Linear peptides Ac-C-εAhx-YKAK₇WK and Ac-CK(C₁₀)-εAhx-YKAK₇WK activate G_i and, to a lesser extent, G_s proteins, whereas star-like peptide [(GRKKRRQRRRPPQ)₂-K-εAhx-C]₂ and dendrimer (NH₂)₃₂(K)₁₆(K)₈(K)₄(K)₂KA preferably G_i proteins. Non-GPCR peptides decrease a little signal transduction but this effect is not receptor-specific. The peptide, derivative of C-ICL3 of G_i-coupled type 1B 5-hydroxytryptamine receptors (5-HT_1BR), selectively activates G_i proteins and disturbs signal transduction via 5-HT_1BR. The peptides, derivatives of C-ICL3 of G_s-coupled receptors such as 5-HT₆R, relaxin receptor RXFP1 and luteinizing hormone/chorionic gonadotropin receptor (LH/CG-R), selectively activate G_s proteins and reduce signal transduction via 5-HT₆R, RXFP1 and LH/CG-R, respectively. It was shown that GPCR-peptides efficiently stimulate the AC activity and GppNHp binding only in the presence of their cognate receptors. Thus, the GPCR-derived peptides interact very selectively with G proteins and in a receptor-specific manner block signal transduction via AC system, in which respect they differ from non-GPCR peptides. GPCR-peptides are novel candidates for the development of selective peptide-based regulators of hormonal signaling systems.