The ventromedial nucleus of the hypothalamus (VMN) first appears using Nissl stains as a bilateral cell group at the base of the diencephalon around embryonic day 16/17 (E16/17) in mice (McClellan et al., 2006). The heterogeneity of the VMN contributes to the many roles it plays in neuroendocrine function. These roles include influencing female sexual behavior, feeding behavior, anxiety/ defensive behavior, and pain (Canteras et al., 1994, Dielenberg and McGregor, 2001, King, 2006). The VMN is loosely categorized into three main regions: dorsomedial, central, and ventrolateral (Saper et al., 1976, Van Houten and Brawer, 1978). The dorsomedial and central regions are characterized by the expression of the transcription factor steroidogenic factor-1 (SF-1), and the ventrolateral region can be characterized by the expression of ERα (Simerly et al., 1990, Dellovade et al., 2000). SF-1 is one critical transcription factor in VMN development as it plays a role in establishing the cytoarchitecture of the nucleus through terminal differentiation (Tran et al., 2003) and distribution of neuronal phenotypes (Dellovade et al., 2000, Davis et al., 2004a). Although SF-1 is important in VMN development, our prior work and work of others suggests that other factors, in particular, gamma-aminobutyric acid (GABA) are also likely to be involved in determining the boundaries of the nucleus by influencing the movement characteristics of migrating neurons. The neurotransmitter GABA has an interesting relationship with the development of the VMN (Tobet et al., 1999). During early stages of development, GABA is synthesized in positions that could provide potential boundary information for the embryonic VMN. GABAergic neurons and fibers surround the embryonic VMN, and towards the end of gestation in mice, GABAergic fibers begin to infiltrate interior regions of the VMN (Tobet et al., 1999). In contrast to the late gestational in-growth of fibers, subunits for GABAA (Dellovade et al., 2001), and GABAB (Davis et al., 2002) receptors are expressed in neurons within the region of the VMN as early as E13 and throughout adulthood. In addition, physiological analyses have been performed as early as E18 and indicate the presence of functional receptors in the developing mediobasal hypothalamus of rats (Obrietan and van den Pol, 1995, Obrietan and van den Pol, 1998). In addition to its role as the major inhibitory neurotransmitter in the adult CNS, GABA is also important in many developmental processes, including cell proliferation (LaMantia, 1995) and neuronal migration (Behar et al., 1996, Behar et al., 1998, Manent and Represa, 2007). GABA’s influence on migration can be mediated through either or both its A and B receptors. In cortical migration, GABAA and GABAB receptors play a role in the formation of cortical layers (Behar et al., 1998). Data to date indicate that the ability of GABA to influence neuronal migration within the VMN is mediated by both ionotropic GABAA (Dellovade et al., 2001) and metabotropic GABAB receptors (Davis et al., 2002). The GABAA receptor agonist muscimol caused a decrease in the percent of neurons moving within the region of the developing VMN. The addition of baclofen, a GABAB receptor agonist, to live tissue slices also caused a dose-dependent decrease in the rate of motion of cells in the region of the VMN (Davis et al., 2002). Baclofen administration did not change the probability of cells moving nor did it have an effect on the angle of cell movement. Based on the Nissl stained gross cytoarchitecture, baclofen did not influence the ability of the VMN to form. The current study further examines the role of GABAB receptors in the development of the embryonic and early postnatal murine VMN and compares the role of these receptors on the development of the neighboring arcuate nucleus (ARC). Mice in which the Thy-1 promoter drives neuron-selective yellow fluorescent protein (YFP) expression (Feng et al., 2000), were utilized for live video microscopy studies in vitro to evaluate the potential role of endogenous GABA acting at GABA receptors on the movement of one population of migrating neurons (Thy-1/YFP expressing neurons) by using GABA receptor antagonists. These mice were used as a tool to follow the movement patterns of fluorescently labeled cells in the regions of the VMN and ARC. It is unknown as to why YFP expression is found in a subset of VMN and ARC neurons at embryonic ages, but the limited pattern of expression is likely due to the insertion site of the transgene, the number of copies incorporated into each line, or the interactions of the transgene elements with flanking DNA (Feng et al., 2000). GABAB receptor knockout mice (Prosser et al., 2001) were utilized to determine the dependence of VMN formation on GABAB signaling during early development. Immunocytochemistry and in situ hybridization were used to identify cell phenotypes for ERα, BDNF (brain derived neurotrophic factor), SF-1, and specific GABAA receptor subunits to determine potential differences in cell position or expression in mice without GABAB receptor signaling.