We have developed a new method for chronic application of electrical stimuli to the rat peripheral nerve in vivo. This method has the following advantages: (1) the amplitude, duration and pattern of stimulation can be adjusted before and during the course of experiment, (2) the set-up allows the animal to move freely during the experiment, and (3) the set-up is constructed from inexpensive, of-the-shelf components that can be reused several times. The new method was used to study the influence of the pattern of muscle activation on acetylcholinesterase (AChE) regulation in extensor digitorum longus (EDL) and tibialis anterior (TA), fast skeletal muscles. Northern blot analysis of the chronically stimulated, fast EDL and TA revealed a rapid decrease of AChE mRNA level to a level typical for a slow, skeletal muscle.