GABAB receptors are the G-protein-coupled receptors for the neurotransmitter γ-aminobutyric acid (GABA). Receptor subtypes are based on the subunit isoforms GABAB1a and GABAB1b, which combine with GABAB2 subunits to form heteromeric receptors. Here, we used a modified bacterial artificial chromosome (BAC) containing the GABAB1 gene to generate transgenic mice expressing GABAB1a and GABA B1b subunits fused to the enhanced green fluorescence protein (eGFP). We demonstrate that the GABAB1-eGFP fusion proteins reproduce the cellular expression patterns of endogenous GABAB1 proteins in the brain and in peripheral tissue. Crossing the GABAB1-eGFP BAC transgene into the GABAB1 -/- background restores pre and postsynaptic GABAB functions, showing that the GABAB1-eGFP fusion proteins substitute for the lack of endogenous GABAB1 proteins. Finally, we demonstrate that the GABAB1-eGFP fusion proteins replicate the temporal expression patterns of native GABAB receptors in cultured neurons. These transgenic mice therefore provide a validated tool for direct visualization of native GABAB receptors.