The influence of an increase or a decrease in the stimulation frequency on tension development during a tetanus was studied in motor units of the rat medial gastrocnemius muscle. These effects were tested in one tetanus evoked at two frequencies of stimulation, a lower immediately followed by a higher one or the reverse. For all fast motor units it was observed that after the first part of a tetanus at a lower frequency of stimulation the tension of the following part, of the better fused contraction, was depressed. This effect was called a tetanic depression. When the lower stimulation frequency was followed by the higher one, the depression was visible in some motor units only whereas in the remaining units a potentiation of the second part of the tetanus was visible. The tetanic depression was larger in fast resistant than in fast fatigable motor units. In slow motor units tetanic depression was not observed. The tetanic depression is a phenomenon which can influence the production of contractile tension by fast motor units.