The molecular chaperone Hsp70 interacts with misfolded proteins and also accumulates in the nucleus during heat shock. Using GFP-Hsp70 and fluorescence recovery after photobleaching, we show that Hsp70 accumulates in the nucleus during heat shock not only because its inflow rate increases but also because of a marked decrease in its outflow rate. Dynamic imaging also shows that GFP-Hsp70 has greatly reduced mobility when it interacts with organelles such as nucleoli in heat-shocked cells or the large inclusions formed from fragments of mutant huntingtin protein. In heat-shocked cells, nucleoplasmic Hsp70 has reduced mobility relative to the cytoplasm, whereas the ATPase-deficient mutant of Hsp70, Hsp70(K71E), is almost completely immobilized both in the nucleoplasm and the cytoplasm. Moreover, the Hsp70 mutant shows reduced mobility in the presence of diffusive huntingtin fragments with expanded polyglutamine repeats. This provides strong evidence that Hsp70 interacts not only with organelles but also with diffusive proteins in the nucleoplasm and cytoplasm during heat shock as well as with diffusive huntingtin fragments.