 CHAPTER 9 Abstract Explaining the emergence of behavioral organization and functional action patterns during ontogeny represents a challenge for developmental science. Using interlimb coordination in the fetal and neonatal rat as a model behavior, this chapter reviews central mechanisms and sensory regulation of spontaneous limb movement, pharmacological induction of locomotor-like behavior, motor learning and memory, and environmental factors that contribute to the construction of organized motor behavior during perinatal development. Recent experiments indicate that action systems in the fetus emerge under the joint influence of neural resources, biomechanical constraints, proprioceptive feedback, and contingencies posed by the intrauterine environment. This research suggests that experience accruing from feedback about motor performance may play a significant role in the perinatal construction of motor behavior. Keywords: behavioral organization, fetus, newborn, ontogeny, locomotion, interlimb coordination, motor performance, limb movement, motor learning, L-DOPA, Quipazine Experience in the Perinatal Development of Action Systems Michele R. Brumley and Scott R. Robinson Introduction Fetal and neonatal rats move and are capable of expressing behavioral organization immediately before and after birth. Yet functional patterns of motor coordination represent a challenge to the motor system, whereby different parts of the ani- mal must move in rather specific spatial and tem- poral relationships to each other. Although motor activity expressed by the fetus often appears ran- dom, uncoordinated, and immature, quantita- tive approaches to examining motor behavior in the fetus have shown that fetal animals exhibit considerable movement organization, including examples of interlimb coordination, before birth (e.g., Kleven, Lane, & Robinson, 2004; Robinson & Smotherman, 1987; Smotherman & Robinson, 1987). How the developing central nervous system (CNS) proceeds from initially expressing only spontaneous movement to shortly thereafter expressing functional patterns of motor behavior remains unresolved. Furthermore, how the devel- oping animal continues to express functional pat- terns of movement with a continuously changing CNS and growing body is a remarkable challenge. Tis developmental challenge in motor control has been called the “calibration problem,” imply- ing that the developing animal must continually modify or recalibrate its motor execution strategy to satisfy the demands of the biomechanical and/or