Ras-related, guanine nucleotide-binding proteins of the Ypt/Rab family play a key role at defined steps in vesicular transport, both in yeast and in mammalian cells. In yeast, Ypt1p has an essential function late in endoplasmic reticulum (ER) to Golgi transport, and the redundant Ypt31/Ypt32 GTPases have been proposed to act in transport through and/or from the Golgi. Here we report that mutant alleles of YPT31 and YPT32, whose gene products have a reduced affinity for GTP, are able to suppress the dominant lethal phenotype of YPT1(N121I). Co-expression of YPT1(N121I) and the suppressor YPT31(N126I) allow essentially undisturbed secretory transport in the absence of the respective wild-type GTPases. Such mutant cells massively overaccumulate 60-100 nm vesicles and are heat sensitive. It appears likely that the mutant GTPases, which are defective in nucleotide binding, compete for the binding of common interacting protein(s). These and other genetic interactions between YPT1, YPT31/32, ARF1 and SEC4 described here strongly support the view that Ypt31p and Ypt32p have a central, Golgi-associated function in anterograde or retrograde transport.