Changes in the sensitivity of the respiratory system to carbon dioxide was studied after administration of NMDA receptor antagonist MK-801 by comparison of the response of the respiratory system with arterial blood pressure in anaesthetized non-inbred albino rats during 5 min hypercapnic (5-6% CO2 in the air) gas mixture inhalation before and 30 min after systemic MK-801 administration. It was established that after NMDA receptors blocking in rats the respiratory response was the same as it was in the control animals. Thus, NMDA receptors blocking is not followed by a significant change in respiratory chemosensitivity to carbon dioxide.