1. The slow-twitch soleus muscle and the fast-twitch extensor digitorum longus muscle (EDL) were denervated and stimulated directly with implanted electrodes for 33-82 days. Four different stimulation patterns were used in order to mimic important characteristics of the natural motor-unit activity in these muscles. In addition, to compare the effects of direct stimulation to other experimental models, some EDLs were stimulated through the nerve or cross-innervated by soleus axons. 2. After 33-82 days of stimulation the contractile properties were measured under isometric and isotonic conditions. 3. 'Native' stimulation patterns could maintain normal contractile speed in both EDL and soleus. In the EDL, normal isotonic shortening velocity was maintained only by a stimulation pattern consisting of very brief trains with an initial short interspike interval (doublet), and not by the other 'native' high-frequency patterns. 4. The contractile properties of both EDL and soleus muscles receiving a 'foreign' stimulation pattern were transformed in the direction of the muscle normally receiving that type of activity. The transformations were not complete, and soleus and EDL muscles stimulated with the same stimulation pattern remained different. This suggests that adult muscle fibres in rat EDL and soleus are irreversibly differentiated into different fibre types earlier in development. 5. The three high-frequency stimulation patterns used differed in their ability to change or maintain various contractile properties in the soleus and the EDL. The results indicate that the following qualities of a stimulation pattern might be of importance for the control of contractile properties: instantaneous frequency, total amount of stimulation, train length, interval between trains and presence of an initial doublet. 6. With the exception of the EDL shortening velocity, changes in contractile speed induced by a 'foreign' stimulation pattern were quantitatively similar to the effects of cross-innervation both in the EDL and the soleus. We thus suggest that the change in activity pattern is the mechanism behind most of the changes induced by cross-innervation.