An electrophysiological method was used to analyze specific features in effecting the reactions of the optimum and pessimum in young puppies. In puppies aged under 16–18 days the greatest values of the action potential in the muscle (gastrocnemius) was recorded by stimulating the nerve with a pulse frequency of 5–12/sec, which corresponds to the optimum of the muscle reaction. The pessimum reaction in the animals of the mentioned young age does not take place even when the frequency of the nerve stimulation reaches 200 and more stimuli/sec. In such a case the neuro-muscular apparatus does not show any signs of fatigue in prolonged stimulation, not only by the optimal but also by a higher frequency. The phenomenon of posttetanic activation is not effected at this age. In puppies aged 16–18 days up to the age of 2–3 months there is a gradual rise of the frequency optimum (first up to 20, and then to 30/sec) and a capacity for true pessimum reaction gradually develops.