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Metamorphosis, a widespread life history strategy in metazoans, allows dispersal and use of different ecological niches through a dramatic body change from a larval stage [ 1 , 2 ]. Despite its conservation and importance, the molecular mechanisms underlying its initiation and progression have been characterized in only a few animal models. In this study, through pharmacological and gene functional analyses, we identified neurotransmitters responsible for metamorphosis of the ascidian Ciona. Ciona metamorphosis converts swimming tadpole larvae into vase-like, sessile adults. Here, we show that the neurotransmitter GABA is a key regulator of metamorphosis. We found that gonadotropin-releasing hormone (GnRH) is a downstream neuropeptide of GABA. Although GABA is generally thought of as an inhibitory neurotransmitter, we found that it positively regulates secretion of GnRH through the metabotropic GABA receptor during Ciona metamorphosis. GnRH is necessary for reproductive maturation in vertebrates, and GABA is an important excitatory regulator of GnRH in the hypothalamus during puberty [ 3 , 4 ]. Our findings reveal another role of the GABA-GnRH axis in the regulation of post-embryonic development in chordates. • The neurotransmitter GABA is a key regulator of Ciona metamorphosis • Gonadotropin-releasing hormone (GnRH) is the downstream neuropeptide of GABA • GABA positively regulates secretion of GnRH through the metabotropic GABA receptor Metamorphosis is a dramatic body change in metazoans that allows the use of different ecological niches. Hozumi et al. show that GABA is a key regulator of metamorphosis in the ascidian Ciona. During this process, GABA, generally thought of as an inhibitory neurotransmitter, positively regulates secretion of the neuropeptide GnRH. [ABSTRACT FROM AUTHOR]