Calcium/calmodulin-dependent kinase II (CaMKII) is an ubiquitous second messenger that is highly expressed in neurons, where it has been implicated in some of the pathways regulating neuronal discharge as well as N-methyl-D-aspartate (NMDA) receptor-mediated synaptic plasticity. The full expression of the mammalian hypoxic ventilatory response (HVR) requires intact central relays within the nucleus of the solitary tract (nTS), and neural transmission of hypoxic afferent input is mediated by glutamatergic receptor activity, primarily through NMDA receptors. To examine the functional role of CaMKII in HVR, KN-93, a highly selective antagonist of CaMKII, was microinjected in the nTS via bilaterally placed osmotic pumps in freely behaving adult male Sprague-Dawley rats for 3 days. Vehicle-loaded osmotic pumps were surgically placed in control animals, and adequate placement of cannulae was ascertained for all animals. HVR was measured using whole body plethysmography during exposure to 10% O2 balance N2 for 20 min. Compared to control rats, KN-93 administration elicited marked attenuations of peak HVR (pHVR) but did not modify normoxic minute ventilation. Differences in pHVR were primarily attributable to diminished respiratory frequency recruitments during pHVR without significant differences in tidal volume. These findings indicate that CaMKII activation in the nTS mediates respiratory frequency components of the entilatory response to acute hypoxia; however, CaMKII activity does not appear to underlie components of normoxic ventilation.