We compared the tissue content, basal phosphorylation, and stretch-induced phosphorylation of the mitogen-activated protein kinase (MAPK) members; extracellular-signal-regulated kinases (ERK 1/2), p38, and c-Jun NH2-terminal kinase (JNK) in the fast-twitch extensor digitorium longus (EDL) and slow-twitch soleus of young adult (6 month), aged (30 month), and very aged (36 month) F344/NNiaHSD X Brown Norway/BiNia (F344/NXBN) rats. The expression and basal phosphorylation of the ERK 1/2, p38, and JNK MAPK proteins were regulated differently with aging in the EDL and soleus. Stretch induced significant phosphorylation of each signaling molecule in both muscle types of young adult and aged animals. In the very aged animals, stretch stimulated ERK 1/2 MAPK phosphorylation; however, EDL stretch failed to induce JNK MAPK phosphorylation, while soleus stretch was unable to induce the phosphorylation of p38 MAPK. The results suggest that skeletal muscle mechanotransduction processes are affected in very aged F344/NXBN rats and that aging alters load-induced signaling in fast- and slow-twitch muscle types differently.