Publisher Summary The material presented in this chapter is developed within the framework of a long-term study of neuropil patterns in the brain stem reticular core and related structures, and is geared toward elucidating certain aspects of the problems of inhibition and facilitation. Three general topics are considered here: (1) some structuro-functional correlates of brain wave maturative patterns with particular reference to cerebral cortex, cerebellum, and reticular formation, (2) the maturation of cortical activation mechanisms, including cortical “following” reactions, and (3) the development of certain discriminative conditional responses, including habituation-like phenomena. The data discussed in this chapter are drawn from a large body of material gathered over several years from a group of approximately 80 kittens chronically implanted within a few hours to a few days of birth. Although the neurophysiological material and related structural correlative data are derived from this source, a larger body of structural information rests on some 12 years of experience with several thousand brains of laboratory animals examined by Golgi and related techniques. Wherever possible, kittens were operated within 3 to 12 h of birth. Minute amounts of intraperitoneal Nembutal (30 mg/kg) in saline were used as basal anaesthesia and supplemented by open drop ether as needed. Under sterile precautions, animals were placed in a specially constructed head-holder, which was suspended within the frame of a standard Johnson stereotaxic apparatus. Three adjustable arms shaped to the bridge of the nose and the mastoid prominences held the head motionless without injury to sensitive tissue of mouth, eyes, or ears.