In this report, we have examined the relationship between the onset of neuronal gene transcription and neuronal development by characterizing expression of the early panneuronal Talpha1 alpha-tubulin promoter in developing neurons. In the peripheral nervous system, detectable expression of a beta-galactosidase transgene driven by the Talpha1 promoter (Talpha1:nlacZ) was coincident with neuronal birth dates, with the exception of sympathetic neuroblasts, which expressed the transgene prior to terminal mitosis. Similarly, in the central nervous system, the onset of beta-galactosidase expression was coincident with neuronal birth dates in most identifiable populations of central neurons. A small subpopulation of transgene-positive cells localized to ventricular zones, but the vast majority was observed in locations consistent with their identification as migrating and/or differentiating neurons. To determine more precisely the temporal relationship between transgene expression and terminal mitosis, we analyzed cultures of cortical progenitors that become postmitotic neurons in vitro. When initially plated, the vast majority of cells consisted of dividing, nestin-positive progenitors. Neurons differentiated from these progenitors as early as 1 day in vitro, as indicated by immunostaining for betaIII-tubulin, a neuron-specific tubulin isotype that is turned on shortly after terminal mitosis. Double-labeling studies showed that Talpha1:nlacZ expression was detectable in the same cells and at approximately the same time as was betaIII-tubulin, indicating that detectable transcription of the Talpha1 alpha-tubulin promoter commences at the time of terminal mitosis, at least in culture. This promoter, therefore, provides a valuable tool for genetic manipulation of early developing neurons in transgenic mice.