Single, functionally isolated motor units were studied in the medial gastrocnemius (MG) muscle of cats and rats. Axons of their motoneurons were stimulated with trains of pulses at frequencies increasing from 1 to 150 Hz and forces developed by muscle fibers were measured and force-frequency curves were compared between species. The following observations were made: (1) the most steep parts of curves (related to unfused tetani of motor units) begun at lower frequencies of stimulations in all types of feline motor units, (2) for fast motor units, the same relative values of force of unfused tetani were achieved at significantly lower frequencies of stimulations in the cat than in the rat. Twitch time parameters of both species influenced the course of force-frequency curves. It was showed that the contraction times of feline units varied in the wide range (21-81 ms), and these units reached 60% of the maximum force at stimulation frequencies between 10 and 38 Hz. On the other hand, contraction times of rat units ranged from 10 to 34 ms, whereas stimulation frequencies necessary to reach 60% of the maximum force varied considerably, from 12 to 65 Hz. The correlations between the above parameters were found for motor units of each species. However, the regression lines drown for the collected population of cat and rat units did not form linear continuity. Thus it seems that interspecies differences in the twitch contraction times do not fully explain different force-frequency relationships in mammalian skeletal muscles.