Different physiological rates of 'tonic' long-term electrical stimulation (rates 5-40 Hz; activity greater than or equal to 50% total time) were delivered to the left-side common peroneal nerve of the cat hind limb. The duration of treatment was 8 weeks, and the animals had previously been subjected to a left-side hemispinalization and dorsal rhizotomy. In the absence of stimulation, these operations had no slowing or weakening effects on peroneal muscle contraction. The minimum two-pulse interval that gave a summation of tension (neuromuscular refractory period) was longer for stimulated than for non-stimulated muscles. Twitches of chronically stimulated muscles had become prolonged by more than 100%. Corresponding changes were found in the tension-frequency relation and in the 'sag'-behaviour of the stimulated muscles. There were no differences between the 'fast' (20 or 40 Hz pulse rates) and the 'slow' (5 or 10 Hz pulse rates) patterns of tonic stimulation with respect to their effects on speed-related muscle properties. Furthermore, during the period of chronic stimulation, the prolongation of twitch contraction time occurred along the same time course for the fast and slow patterns of tonic treatment. All chronically stimulated muscles had become weaker than normal. In comparison to the slow patterns, the present fast patterns of long-term activation caused (1) a smaller amount of decline in maximum muscle force, (2) a smaller twitch: tetanus ratio, and (3) the retention of a normal amount of post-tetanic potentiation of twitch size (decreased by the slow patterns). When tested by a series of 40 Hz bursts, force was better maintained in chronically stimulated muscles than in normal ones. These effects on fatigue resistance were the same for the fast and slow patterns of long-term activation. In peroneus longus muscles contralateral to the side of chronic activation, an evident impairment had commonly occurred in the capability to maintain force during tetani at the high rates needed for a maximum tetanic contraction. The results are discussed in relation to problems concerning the long-term effects of motoneuronal activity patterns on the contractile properties of their muscle units.