The development of the central nervous system in the Drosophila embryo is initiated by the acquisition of neural potential by clusters of ectodermal cells, promoted by the activity of proneural genes. Proneural gene function is antagonized by neurogenic genes, resulting in the realization of the neural potential in a single cell per cluster. To analyse the relationship between proneural and neurogenic genes, we have studied, in specific proneural clusters and neuroblasts of wild-type and neurogenic mutants embryos, the expression at the RNA and protein levels of lethal of scute, the most important known proneural gene in central neurogenesis. We find that the restriction of lethal of scute expression that accompanies the restriction of the neural potential to the delaminating neuroblast is regulated at the transcriptional level by neurogenic genes. These genes, however, do not control the size of proneural clusters. Moreover, available antibodies do not provide evidence for an hypothetical posttranscriptional regulation of proneural proteins by neurogenic genes. We also find that neurogenic genes are required for the specification of the mesectoderm. This has been shown for neuralized and Notch, and could also be the case for Delta and for the Enhancer of split gene complex. Neurogenic genes would control at the transcriptional level the repression of proneural genes and the activation of single-minded in the anlage of the mesectoderm.