This article summarizes a number of lines of investigation of rRNA gene expression that are ongoing in the laboratory. These studies focus on mouse and frog, two distant vertebrate species. One major conclusion is that the basic properties of rRNA gene expression appear remarkably well conserved in evolution, with only relatively minor perturbations between frog and mouse, contrary to the common interpretation of the species-selectively between mouse and human rDNA transcription (e.g., 1). This is true both for the process of rDNA transcription and for the subsequent rRNA processing event.