GABA(B) receptors are G-protein-coupled receptors that mediate slow onset and prolonged effects of GABA in the central nervous system (CNS). While they appear to influence developmental events, depending on where they are found at a synapse, little, if anything, is known as to the expression of GABA(B1) and GABA(B2) receptor mRNAs during the early developmental stages. We used in situ hybridization and RNase protection assays (RPA) to investigate the early fetal expression of GABA(B1) and GABA(B2) receptor mRNAs on the development of the rat CNS. Our in situ studies defined a pattern of early and strong GABA(B1) receptor mRNA expression in the spinal cord, medullar and cerebral cortex neuroepithelium of discrete brain regions on gestational day (GD) 11.5. On GD 12.5, GABA(B1) receptor mRNAs were found in the hippocampal formation, cerebral cortex, intermediate and posterior neuroepithelium, and the pontine neuroepithelium of whole brain. RPA results showed GABA(B1) receptor mRNA was intensely expressed on GD 11.5 and GD 12.5, when it was first detected in the ganglia, thalamus, and cerebellum. However, GABA(B2) receptor mRNA was not detected on GD 10.5, 11.5, or 12.5. We suggest that GABA(B1) receptor might have a role in the early fetal brain and spinal cord during pre- and post-synaptogenesis, neuronal maturation, proliferation, and migration, and may be more important than the GABA(B2) receptor in the early development of the rat CNS.