Helicobacter pylori induces signaling cascades leading to changes in cytoskeleton and an inflammatory response. Information on the morphological changes and cytoskeletal rearrangements induced by attachment of the bacterium is contradictory and signal transduction pathways are not well known. Since rho family of small GTPases is known to mediate cytoskeletal response to various extracellular stimuli, and is also involved in several other important signal transduction pathways, we have investigated the role of rac and cdc42 in H. pylori-induced cytoskeletal changes in cultured carcinoma AGS cells. AGS cells grown with serum expressed actin filaments in the form of short stress fibers and thin network at the edges, which were depolymerized by removal of serum. In serum-starved cells both type I and type II strains of H. pylori induced formation of actin filaments and lamellipodia-like structures. Microinjection of active rac induced similar changes, but injection of inactive rac prevented the effects of H. pylori, while active or inactive cdc42 did not have any significant effect. Cytoskeletal effects of H. pylori were inhibited by actinomycin D, but not completely by cycloheximide. These results indicate that rac activation is involved in signal transduction cascade leading to cytoskeletal reorganization induced by H. pylori and that gene activation and synthesis of new proteins is necessary in this process.