The movement of newly synthesized proteins in the constitutive secretory pathway, from their site of synthesis in the endoplasmic reticulum to the cell surface or to intracellular destinations, requires an orderly sequence of transport steps between membrane-bound compartments. Until recently, the trafficking and secretion of proteins through this pathway was thought to occur as a relatively automatic, unregulated series of events. Recent studies show that protein trafficking in the constitutive secretory pathway requires G T P hydrolysis by families of GTP-binding proteins (G proteins), which at multiple steps potentially provide regulation and specificity for protein trafficking. M any monomeric G proteins are known to be localized and functional on membrane compartments in the constitutive secretory pathway. Now, members of the heterotrimeric G protein family have also been localized on intracellular membranes and compartments such as the Golgi complex. We have studied the localization and targeting of G a subunits to distinct membrane domains in polarized epithelial cells. The distribution of different G a subunits on very specific membrane domains in cultured epithelial cells and in epithelial cells of the kidney cortex, is highly suggestive of roles for these G proteins in intracellular trafficking pathways. One of these G protein subunits, Gai-3, was localized on Golgi membranes. Studies on L L C -P K i cells overexpressing Gcti.3 provided evidence for its functional role in regulating the transport o f a constitutively secreted heparan sulfate proteoglycan through the Golgi complex. Inhibition or activation of heterotrimeric G proteins by pertussis toxin or by aluminium fluoride respectively, have provided further evidence for regulation of intracellular transport by pertussis toxin-sensitive G proteins. Although the functions of Golgi-associated G proteins are not yet understood at the molecular level, heterotrimeric G proteins have been implicated in the binding of cytosolic coat proteins and vesicle formation on Golgi membranes. Future studies will elucidate how multiple G proteins, of both the heterotrimeric and monomeric families, are involved in the regulation of Golgi function and protein trafficking in the secretory pathway