Induction of the protective heat shock proteins (Hsps), and of Hsp72 in particular, has been reported to be decreased in certain tissues from aged animals. To determine if both fast and slow skeletal muscles from aged animals demonstrate an altered ability to induce and accumulate Hsp72, adult (age, 6 months) and aged (age, 20 months) Fischer 344 rats were subjected to heat stress. At selected times (0, 1, 3, and 24 hours) after a 10-minute, 41°C heat stress, fast (white gastrocnemius [WG]) and slow (soleus) skeletal muscles were examined for either heat shock transcription factor (HSF) activation (trimerization and DNA-binding activity) or Hsp72 content using electrophoretic gel mobility shift assays and Western blotting, respectively. Immediately after heat stress, the level of HSF activation between aged and adult animals was similar for both muscles. HSF activation was undetectable at 1 and 3 hours after heat stress in all cases. Twenty-four hours after heat stress, Hsp72 content in the WG muscles from both aged and adult animals was significantly increased compared with unstressed, age-matched controls (P < 0.05). In contrast, perhaps because of their high constitutive Hsp72 levels, soleus muscles from both aged and adult animals did not demonstrate a significant increase in Hsp72 content after heat shock, but there was a trend toward increased levels. Hsp72 content in both the soleus and WG muscles demonstrated no significant differences between adult and aged animals in either the unstressed state (controls) or after heat shock. These results suggest that skeletal muscles from aged animals are capable of inducing the heat shock response and accumulating Hsp72.