Growth-blocking peptide (GBP) was first identified in the hemolymph of the host armyworm, Pseudaletia separata, whose growth is halted in the last instar larval stage by parasitization with the parasitoid wasp, Cotesia kariyai. Studies on the mechanism of growth retardation by GBP revealed that GBP titers in hemolymph fluctuate synchronously with dopamine levels: GBP and dopamine peaks coincide with each larval molt period, during which larvae temporarily cease moving and feeding. The fact that GBP induced the elevation of dopamine concentration in the hemolymph was demonstrated in armyworm larvae by GBP injection. Dopamine elevation was also observed in insects exposed to various stress conditions such as parasitization and chilling. These results, together with the fact that dopamine itself inhibits larval growth, indicate that GBP induces growth retardation via the elevation of dopamine levels. Further, we demonstrated that the diapause-inducing influence of short day length also elevates dopamine concentrations in hemolymphs and the brain-central nervous system (Br-CNS) of the cabbage armyworm, Mamestra brassicae. The elevation of dopamine levels contributes to the onset of pupal diapause. We therefore proposed that a GBP-dopamine system contributes to the control of growth rates in insects. Recent studies by ourselves and other laboratories have found more than 10 homologous peptides in various insect species. These peptides have diverse functions: larval growth retardation, paralysis induction, cell proliferation, immune cell stimulation and cardioacceleration. As demonstrated, GBP itself exerts most of these functions, so it is reasonable to conclude that GBP and GBP-like peptides widely present in insects should be regarded as insect cytokines with a variety of functions.