We have recently found, by use of the rhoA p21 purified from bovine aortic smooth muscle, that it is similarly post-translationally processed as described for ras p21s: it is first geranylgeranylated at the cysteine residue in the C-terminal region followed by removal of the three C-terminal amino acids and the subsequent carboxyl methylation of the revealed C-terminal cysteine residue. In the present study, we investigated the function(s) of these post-translational modifications of the C-terminal region of rhoA p21 by use of the rhoA p21s purified from bovine aortic smooth muscle and rhoA p21-overexpressing Escherichia coli since the bacterial protein was not modified with a geranylgeranyl moiety. Bovine rhoA p21 bound to plasma membranes and phosphatidylserine-linked Affigel, but bacterial rhoA p21 did not bind to them. The inhibitory GDP/GTP exchange protein for rhoA p21, named GDP dissociation inhibitor (GDI), made a complex with the GDP-bound form of bovine rhoA p21 and thereby inhibited the dissociation of GDP from and the subsequent binding of GTP to it. However, rho GDI neither made a complex with the GDP-bound form of bacterial rhoA p21 nor affected these reactions of the bacterial protein. The stimulatory GDP/GTP exchange protein for rhoA p21, named GDP dissociation stimulator (GDS), stimulated the dissociation of GDP from bovine rhoA p21, but was inactive for the bacterial protein. In contrast, the GTPase activating protein for rhoA p21 is active not only for bovine rhoA p21 but also for the bacterial protein. These results suggest that the post-translational modifications of the C-terminal region of bovine rhoA p21, most presumably the geranylgeranylation, which are absent in bacterial rhoA p21, play important roles in its interaction with membranes and the stimulatory and inhibitory GDP/GTP exchange proteins but not with the GAP.