Abstract Rho-related proteins are members of the ras superfamily of small GTP-binding proteins. Their function in fibroblasts has been analysed using microinjection of living cells. Rho appears to link plasma membrane receptors to the assembly of focal adhesions and actin stress fibres. The closely related protein rac, on the other hand, links receptors to the polymerization of actin at the plasma membrane to form membrane ruffles and pinocytotic vesicles. In phagocytic cells, rac has been shown to be required for activation of a membrane-bound NADPH oxidase in response to receptor activation. These systems provide the basis for a working model for the mechanism of action of the rho family of small GTPases.