Opioid-induced respiratory effects include aspiration and difficulty swallowing, suggesting impairment of the upper airways. The pontine Kölliker-Fuse (KF) controls upper airway patency and regulates respiration, in particular the inspiratory/expiratory phase transition. Given the importance of the KF in coordinating respiratory pattern, the mechanisms of mu opioid receptor activation in this nucleus were investigated at the systems and cellular level. In anesthetized, vagi-intact rats, injection of opioid agonist DAMGO or [Met(5) ]enkephalin (ME) into the KF reduced respiratory frequency and amplitude. The mu opioid agonist DAMGO applied directly into the KF of the in situ arterially perfused working heart-brainstem preparation of rat resulted in robust apneusis (lengthened low amplitude inspiration due to loss of post-inspiratory drive) that was rapidly reversed by the opioid antagonist naloxone. In brain slice preparations, activation of mu opioid receptors on KF neurons hyperpolarized a distinct population (61%) of neurons. As expected, the opioid-induced hyperpolarization reduced the excitability of the neuron in response to either current injection or local application of glutamate. In voltage-clamp recordings the outward current produced by the opioid agonist ME was concentration-dependent, reversed at the potassium equilibrium potential and was blocked by BaCl2 , characteristics of a G protein-coupled inwardly rectifying potassium (GIRK) conductance. The clinically used drug morphine produced an outward current in KF neurons with similar potency to morphine-mediated currents in locus coeruleus brain slice preparations. Thus, the population of KF neurons that are hyperpolarized by mu opioid agonists are likely mediators of the opioid-induced loss of post-inspiration and induction of apneusis. This article is protected by copyright. All rights reserved.