We have analysed the expression patterns of two human genes coding for two different rab GDIs, rab GDI α/XAP-4 and rab GDI β, proteins involved in the regulation of vesicle-mediated cellular transport. The gene sequences are extremely conserved in evolution, with substantial homology preserved across three eukaryotic kingdoms. Although the sequence homology between the two human rab GDIs studied is very high, their expression patterns are completely different. The Northern blot analysis and in situ hybridization to sections of mouse embryos and postnatal tissues have revealed that the tab GDI α/XAP-4 is expressed predominantly in neural and sensory tissues and may thus serve a specific function in neural signal transmission. In contrast to tab GDI α/XAP-4, the human rab GDI β is expressed ubiquitously.