In Nembutal-anesthetized and spontaneously breathing rats, a total of 226 respiratory neurons were recorded in the medulla extending from the caudal end of the facial nucleus to 1 mm caudal to the obex. They were classified into inspiratory (I) and expiratory (E) neurons by their temporal relationships to diaphragm EMGs. One hundred and seventeen I and 108 E neurons were identified. I and E neurons were further classified into augmenting, decrementing, and other types based on their firing patterns. Almost all the respiratory neurons recorded were located around the nucleus ambiguus and the nucleus retroambigualis, corresponding to the ventral respiratory group (VRG) of the cat. On the other hand, only a few respiratory neurons were identified around the ventrolateral nucleus of the solitary tract, corresponding to the dorsal respiratory group of the cat. In the VRG, 3 subgroups were distinguished rostrocaudally. One group of E neurons was located ventrally to the rostral part of the nucleus ambiguus, presumably corresponding to the Bötzinger complex defined in the cat. Another group of E neurons extended caudally beyond the obex, from the caudal portion of the nucleus ambiguus through the nucleus retroambigualis. Between these two groups of E neurons, an assembly of predominantly I neurons existed in the vicinity of the nucleus ambiguus. These characteristics of distributions were basically similar to those of the VRG of the cat.