The intracellular distribution of the major Drosophila heat-shock protein hsp70 was determined by indirect immunofluorescence with monoclonal antibodies. During heat shock the protein concentrates strongly in nuclei while a small quantity remains cytoplasmic. During recovery hsp70 leaves the nuclei and becomes distributed throughout the cytoplasm. With a second heat shock it is rapidly transported back into the nucleus. Nuclear translocation depends not on the temperature per se, but on the physiological state of the cell since it also occurs after exposure to an anoxic atmosphere at normal temperatures. We also provide evidence that hsps protect cells from the toxic effects of anoxia, as well as heat, and conclude that nuclear translocation of hsp70 is related to its function in protecting the organism from both forms of environmental stress.