Fast muscles of rodents characteristically differ from their slow- twitch counterparts by exhibiting high levels of G4, i.e., the tetrameric acetylcholinesterase (AChE) molecular form. Converging evidence suggests that this additional G4 pool is specifically regulated by the type of activity actually performed by the muscle. This hypothesis was tested by studying the effect of a chronic increase in neuromuscular activity on the AChE content and distribution of molecular forms of functionally antagonist rat hindlimb muscles. They included the fast ankle extensors gastrocnemius (GAST) and plantaris (PL), the fast ankle flexors tibialis anterior (TA) and extensor digitorum longus (EDL), as well as the slow-twitch soleus (SOL). Neuromuscular activity was enhanced by subjecting the rats to a 12-week training program consisting of repeated sessions of prolonged endurance running on a rodent treadmill. This exercise regimen preferentially affected the G4 pool characterizing fast muscles which exhibited marked and opposite changes according to the functional role of the muscles. The amount of G4 was increased by more than 50% in the ankle extensors GAST and PL, which play a dynamic role, and reduced by about 40% in the ankle flexors TA and EDL, which exhibit a predominant tonic activity during running. The asymmetric forms A12 and A8 were slightly elevated in the fast muscles. In the case of the slow-twitch SOL, running training resulted in a small, nonspecific decrease in AChE content which affected most of the molecular forms. These data indicate that the size of the G4 pool characteristic of fast muscles depends on the type, dynamic or tonic, of activity actually performed. The present results support the conclusion that this G4 pool fulfills a specific and essential function, distinct from that of A12.