There is a superfamily of small GTP-binding proteins in which more than forty members are included (Bourne et al. 1991; Hall 1990; Takai et al. 1992). They exhibit GDP/GTP-binding and GTPase activities and have two interconvertible forms: GDP-bound inactive and GTP-bound active forms (Fig. 1). The GDP-bound form is converted to the GTP-bound form by the GDP/GTP exchange reaction which is regulated by GDP/GTP exchange proteins (GEPs). The GDP/GTP exchange reaction is initiated by the dissociation of GDP from the GDP-bound form of small GTPase followed by the association of GTP to the guanine-nucleotide-free form. The GTP-bound form is converted to the GDP-bound form by the GTPase reaction which is stimulated by the GTPase activating protein (GAP) (McCormick 1990). In our laboratory, we have isolated two types of GEP: a stimulatory type named guanine nucleotide dissociation stimulator (GDS) and an inhibitory type named guanine nucleotide dissociation inhibitor (GDI) (Takai et al. 1992). We have purified smg GDS, rho GDI and smg p251 GDI to homogeneity from mammalian tissues, cloned their cDNAs, and determined their primary structures, but we have only partially purified rho GDS. In this review article, we will describe these GEPs.