The ventromedial nucleus of the hypothalamus (VMH) in mice first emerges as a histologically distinct cell cluster around embryonic day 17 (E17). The earliest known marker for cells destined to form the VMH is the orphan nuclear receptor, steroidogenic factor 1 (SF‐1), which can be detected in the hypothalamic primordium by E11. Strikingly, the VMH is absent in newborn SF‐1 knockout mice, suggesting that SF‐1 is essential for the development of VMH neurons. We reported previously that the VMH can be identified before it emerges as a histologically distinct nucleus (i.e., at E13) by the exclusion of cells that are immunoreactive for both γ‐aminobutyric acid (GABA) and the synthetic enzyme, glutamic acid decarboxylase (GAD67). Subsequently, by E15, the developing VMH is demarcated further by cells that are immunoreactive for neuropeptide Y, estrogen receptor α (ERα), and galanin. It is noteworthy that the normal exclusion of GABA from the developing VMH is not seen in SF‐1 knockout mice, and cells that are immunoreactive for neuropeptide Y, ERα, and galanin also are distributed aberrantly in this region. Thus, the absence of SF‐1 profoundly affects the cellular architecture of the VMH from early stages in its formation. These data suggest that, directly or indirectly, SF‐1 plays important roles in determining the distribution of cells in the mediobasal hypothalamus. J. Comp. Neurol. 423:579–589, 2000. © 2000 Wiley‐Liss, Inc.