The influence of an exogenous nitric oxide (NO) donor, sodium nitroprusside, and endogenous NO biosynthesis substrate, L-arginine, on integrated respiratory rhythm recorded from the phrenic nerve of the semi-isolated medullo-spinal preparations of the 3-to-4-days old rats was studied. Applications of 10.0 mcM of these drugs led to significant increase in the duration and amplitude of the inspiratory discharges (ID) in the phrenic nerve and to decrease of ID frequency. After the rostral ventrolateral (RVL) medulla (projectionally corresponds to "M" chemosensitive zone) had been separated by transection, such applications began to evoke a rise of ID frequency without significant changes of the duration and amplitude. Thus, we found the involvement of NO in the tonic inhibition of the respiratory rhythm by RVL medullary structures in the neonatal rats.