We investigated intracellular translocation of hsp70 and total protein synthesis during heating at mild temperatures in NRK (normal rat kidney) cells. When cells were heated at 41 degrees C, the hsp70 was translocated into the nuclei/nucleoli, then returned gradually to the cytoplasm during heating. Concomitantly, total protein synthesis first decreased to 80% of the control level, then recovered by 2 hr. This indicates the acquisition of translational thermotolerance during heating at 41 degrees C. The return of hsp70 to the cytoplasm seems to be related to the recovery of total protein synthesis. Similar relationships were obtained at 42 degrees C heating in the presence of glycerol. These results suggest that translocation of hsp70 into the nuclei/nucleoli is important for the recovery of protein synthesis (acquisition of translational thermotolerance) during heating at mild temperatures.