For the first time, insulinlike growth factor I (IGF-I), like insulin (Pertseva et al., Comp Biochem Physiol 112: 689-695, 1995), was shown to exercise a GTP-dependent stimulating action on adenylyl cyclase (AC; EC 4.6.1.1.) activity in the muscle tissues (membrane fraction) of mammal (rat) and mollusc (Anodonta cygnea). By studying the mechanism of the effect of peptides with selective inhibitors of tyrosine kinase activity, tyrphostin 47 (RG50864, 3,4-dihydroxy-alpha-cyanothiocinnamamide) and genistein (4,5,7-trihydroxyisoflavone), it was found that receptor tyrosine kinase is involved in this action. The data obtained suggest that the stimulating effect of insulin and IGF-1 is produced via the following signalling system: receptor tyrosine kinase --> stimulatory G-protein --> AC. Thus, the existence of a novel signalling pathway of transduction of signals generated by insulin and related peptides was hypothesised.