In this brief review, I focused on the abdominal expiratory motor pattern in the rat. In the vagotomized adult rat, hypercapnic acidosis evoked two patterns of the abdominal expiratory activity; one with low amplitude expiratory discharge (E-all activity) that persisted throughout the expiratory phase, and another with late expiratory and high amplitude bursts (E2 activity) superimposed on the E-all activity. The E-all activity appeared from milder acidosis than the E2 activity. In the anesthetized, vagotomized or vagus-intact neonatal rats, abdominal muscles often showed not only E2 activity but also a smaller additional burst occurred just after the termination of diaphragmatic inspiratory activity (E1 activity). Since this E1 activity is rarely observed in the adult rat, the abdominal respiratory motor pattern likely changes during postnatal development. Under light anesthesia, vagal afferent feedback shortened the respiratory cycle period due to shortening of the expiratory duration. Further decrement in depth of anesthesia changed the biphasic E2+E1 abdominal motor activity pattern to E-all activity pattern in the vagus-intact neonatal rat. Since this E-all activity was typically observed with short cycle period in the vagus-intact neonatal rat, relation with the E-all activity in the vagotomized adult rat remained unknown. The vagal feedback should have roles not only in setting the cycle period short but also shaping the expiratory motor pattern in the neonatal rat. Although abdominal muscles in the in vitro preparation from neonatal rat also showed biphasic E2+E1 activity, E2 activity was shorter and/or smaller than the E1 activity.