Previous experiments charted the development of behavioral arousal in postnatal mice. From Postnatal Day 3 (P3) to Postnatal Day 6 (P6) mice (a) become significantly more active, "arousable"; and (b) in large reticular neurons, nucleus gigantocellularis (NGC), patch clamp recordings reveal a significantly increased ability to fire high frequency trains of action potentials as are associated with elevated cortical arousal. These action potential trains depend on delayed rectifiers such as Kv2.1. Here we report tracking the development of expression of a delayed rectifier, Kv2.1 in NGC neurons crucial for initiating CNS arousal. In tissue sections, light microscope immunohistochemistry revealed that expression of Kv2.1 in NGC neurons is greater at day P6 than at P3. Electron microscope immunohistochemistry revealed Kv2.1 labeling on the plasmalemmal surface of soma and dendrites, greater on P6 than P3. In brainstem reticular neuron cell culture, Kv2.1 immunocytochemistry increased monotonically from Days-In-Vitro 3-10, paralleling the ability of such neurons to fire action potential trains. The increase of Kv2.1 expression from P3 to P6, perhaps in conjunction with other delayed rectifier currents, could permit the ability to fire action potential trains in NGC neurons. Further work with genetically identified NGC neurons is indicated.