The expression of recently isolated mammalian ras-related ral, rho, and rab genes was examined in adult mouse tissues. Most of the genes studied were transcribed into two major messenger RNAs. The transcription of each gene appeared to be regulated in a complex manner with a tissue-specific modulation of the two transcripts. One member of the rab gene family, rab3, showed an RNA expression restricted to brain-tissues. Ral, rho, and three other rab genes had a more ubiquitous expression in murine tissues. However, the expression level of each gene showed a high degree of variation depending upon the organ. Among all the members of the enlarged ras gene family examined so far, the rab3 gene is the first example showing an expression restricted to a distinct organ.