The intensity of the monosynaptic spinal cord reflex was studied in rats with prolonged severe diabetes mellitus and with that of moderate severity, of short duration. Methods of H-reflex and M-response of the motor nerves and muscles were used. The number of motor neurons taking part in the reflex proved to be less in diabetic animals; rhythmic H-reflex was depressed. These disturbances were primarily connected with the functional changes of the primary afferent terminals of the segmental spinal cord level. In animals with moderately severe diabetes the operative mediator fraction was readily exhausted and the mediator emission decreased. Besides, animals with a severe form of alloxan diabetes displayed postsynaptic disturbances. A positive correlation between the extent of the H-reflex depression and the severity (duration) of alloxan diabetes was demonstrated.