It’s well known that sympathetic and sensory neurons are affected in the early stages of diabetes mellitus (DM). However, the functional disorders that occur in neurons of the superior cervical ganglion (SCG) under the conditions of DM remain insufficiently studied. Therefore, the aim of this study was to evaluate the effect of streptozotocin-induced diabetes mellitus (DM) of the rats on action potentials (AP) recorded in the superior cervical ganglion`s (SCG) neurons. Rats with blood sugar level more than 30 mM were taken into experiment. The SCG of healthy control rats (n=12), rats at week 4 (n=9), and rats at week 12 after streptozotocin injection (n=9) were studied in vitro. AP of the SCG neurons were registered by the microelectrode technique. Neurons of the SCG were stimulated directly with 150 ms depolarizing current in pulse of 100 pA. The AP parameters of 36 SCG neurons of control rats were alternately compared with the corresponding AP parameters of 22 neurons of rats at week 4 and 30 SCG neurons of rats at week 12 after streptozotocin injection. The data obtained demonstrate that the AP amplitude and overshoot of AP, maximum rise and fall rates, and afterhyperpolarization amplitude significantly decreased at 12 weeks after DM induction. At the same time, the rheobase value significantly increased, this may indicates decreasing of the neurons plasma membrane excitability. Only the AP maximum rate of fall decreased statistically significant at week 4, the maximum rate of rise had an insignificant tendency to decrease at that time. However, the resting membrane potential and excitation threshold didn’t change even at 12 weeks after the injection. Thus, functional disorders of rat SCG neurons were appeared at a quite late stage of DM. The differences in AP parameters may result from neurons’ membrane ionic conductivity alterations, decreasing of its excitability and reducing ion channels efficiency in later stages of DM. This suggests that SCG is an important target of pathophysiological disorders caused by DM.