A number of basic helix‐loop‐helix containing proteins have been shown to be required for neural development at different sites or times. Here, Relax, a novel rat basic helix‐loop‐helix transcriptional regulator, has been isolated and characterized. Analysis of the temporal and spatial distributions shows that the Relax transcripts are detected exclusively in the central nervous system, in discrete regions from embryonic day 11.5 to 18.5. Most strikingly, Relax is expressed along two major boundaries that define the longitudinal axis in the spinal cord and the hindbrain and is a marker of the anterior tip of this axis in the forebrain. Relax‐expressing cells are strictly localized in the ventricular zone of the neural tube, where neural progenitors originate. This unique pattern of expression suggests that Relax is involved in neural fate determination. J. Neurosci. Res. 48:146–158, 1997. © 1997 Wiley‐Liss, Inc.