Gamma-amminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system of vertebrates, serves as an autocrine/paracrine signaling molecule during development, modulating a number of calcium (Ca(2+))-dependent processes, including proliferation, migration, and differentiation, acting via 2 types of GABA receptors (GABARs): ionotropic GABA(A)Rs and metabotropic GABA(B)Rs. Here, we demonstrate that mouse embryonic stem cells (mESCs), which possess the capacity for virtually unlimited self-renewal and pluripotency, synthesize GABA and express functional GABA(A)Rs and GABA(B)Rs, as well as voltage-gated calcium channels (VGCCs), ryanodine receptors (RyRs), and inwardly rectifying potassium (GIRK) channels. On activation, both GABAR types triggered synergistically intracellular calcium rise. Muscimol (a GABA(A)R agonist) induced single Ca(2+) transients involving both VGCC-mediated Ca(2+) influx and intracellular stores, while baclofen (a GABA(B)R agonist) evoked Ca(2+) transients followed by intercellular Ca(2+) waves and oscillations that were resistant to antagonists and entirely dependent on Ca(2+) release from intracellular stores. Prolonged treatment with muscimol slightly inhibited, while baclofen or SR95531 (a GABA(A)R antagonist) significantly facilitated, mESC proliferation. GABA(A)R-specific ligands also induced morphological and gene expression changes indicating a differentiation shift. Our data suggest that the interplay between GABARs and downstream (coupled) effectors differentially modulates mESC proliferation/differentiation through selective activation of second messenger signaling cascades.-Schwirtlich, M., Emri, Z., Antal, K., Máté, Z., Katarova, Z., Szabó, G. GABA(A) and GABA(B) receptors of distinct properties affect oppositely the proliferation of mouse embryonic stem cells through synergistic elevation of intracellular Ca(2+).