Mechanisms by which attainment of specific body sizes trigger developmental transitions to adulthood (e.g. puberty or metamorphosis) are incompletely understood. In Drosophila, metamorphosis is triggered by ecdysone synthesis from the prothoracic gland (PG), whereas growth rate is increased by insulin/insulin growth factor signalling (IIS). Transgene-induced activation of PI3K, the major effector of IIS, within the PG advances the onset of metamorphosis via precocious ecdysone synthesis, raising the possibility that IIS triggers metamorphosis via PI3K activation in the PG. Here we show that blocking the protein kinase A (PKA) pathway in the insulin producing cells (IPCs) increases IIS. This increased IIS increases larval growth rate and also advances the onset of metamorphosis, which is accompanied by precocious ecdysone synthesis and increased transcription of at least one ecdysone biosynthetic gene. Our observations suggest that IIS is regulated by PKA pathway activity in the IPCs. In addition, taken together with previous findings, our observations are consistent with the possibility that, in Drosophila, attainment of a specific body size triggers metamorphosis via the IIS-mediated activation of PI3K and hence ecdysone synthesis in the PG.