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Incubation of cultured cells of mouse C3H-2K fibroblastic cell line and other mammalian cell lines at 42.0–43.0 °C for 30 min or longer caused disintegration of normal actin structures including stress fibers, and induced formation of intranuclear actin paracrystal-like structures, called actin rods. When cells exposed to the elevated temperatures were shifted back to 37 °C, normal actin structures were regained. Pretreatment of cells at moderately high temperatures such as 38.5 °C inhibited formation of the actin rods upon subsequent exposure to 42.0 °C. Neither microtubules nor intermediate filaments were disrupted by the heat treatment. Several heat shock proteins were found to be synthesized under the conditions where actin rods were induced. However, there is no causal relationship between two cellular events, the induction of intranuclear actin rods and the synthesis of heat shock proteins.