Hydrogen peroxide, which was shown to trigger the heat-shock response by activating the immediate binding of the heat-shock factor to DNA heat shock regulatory elements in the promoter of heat-shock genes of Drosophila cells, has also been reported to enhance the synthesis of actin. We show here that very short and transient H2O2 treatments, from 1 s to 2 min, are sufficient to induce an increase of actin synthesis. This increase becomes apparent 2 to 3 h after the short H2O2 treatment. It is inhibited if actinomycin D is present during the short H2O2 treatment. An increase of actin synthesis was also observed during the recovery period after two other stresses: reoxygenation after anoxia and ethanol treatment. The synthesis of two cytoskeletal proteins, tubulin and a 46-kDa insoluble protein of the intermediate filament fraction, was also slightly increased by H2O2 in Drosophila cells, but this increase was not actinomycin D-dependent. H2O2 does not provoke the translocation of the 46-kDa protein to the nuclear fraction as does heat shock. The very rapid stimulation of actin synthesis by H2O2 and the involvement of cytoskeletal elements in many stress situations suggest that actin may play a key role in the response to external stimuli.