The gaseous modulator nitric oxide (NO) is synthesized in the vestibular epithelium under normal and pathological conditions. The influence of donor NO 3-morpholino-sydnonimine hydrochloride (SIN-1) on the function of metabotropic glutamate receptors was studied on the isolated vestibular apparatus of the frog, using the method of external perfusion and the recording of the impulse activity of the nerve contacting with the posterior semicircular canal. SIN-1 (0.1–100 μM) increased the level of background impulse activity of the afferent fibers. SIN-1 (1 and 10 μM) reduced the amplitude of the evoked response of the agonist of metabotropic glutamate receptors (±)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD). The inhibitory effect was reversible and did not depend on the concentration of SIN-1. The results indicate that NO can modulate the function of metabotropic glutamate receptors, modifying the afferent flow from the vestibular apparatus to the central nervous system.