We studied the effects of the non-NMDA antagonist NBQX and of the NMDA antagonist dizocilpine (MK-801), administered separately or together, on the respiratory function of conscious cats. NBQX (20 mg kg-1) did not affect minute ventilation nor the timing of inspiratory and expiratory phases, but the addition of a small dose of dizocilpine (0.15 mg kg-1) induced inspiratory pauses and respiratory arrest in the inspiratory phase (apneusis). Similarly, larger doses of either NBQX or dizocilpine did not induce apneusis but the addition of a small dose of the other compound provoked an apneusis. Thus, a blockade of either non-NMDA or NMDA receptors is well tolerated, but the combined blockade of both receptor types severely disrupts the respiratory function in the cat.