Autonomic nervous functions are considered to be influenced by the system controlling respiratory function, and vice versa. Recently, it became apparent that there is an interaction between the nervous systems controlling circulatory and respiratory functions, but anatomical knowledge about these control systems is still scanty. To compensate this lack of anatomical knowledge about the location of respiratory neurons, supraspinal structures projecting to the phrenic motor nucleus were studied using rats for purpose of comparison with our previous findings in the cat. After the microinjection of a small amount of horseradish peroxidase (HRP) into the phrenic motor nucleus, which was physiologically identified, some 2151 HRP-labeled cells were identified in the 5 selected experiments. These cells were distributed in the medulla (88.7%) and pons (11.3%). The majority were concentrated in and around the two nuclei involved in the cardiovascular control system, i.e. the nucleus para-ambiguus (55.5%) which is known as a pressor-tachycardia-apnea response zone and nucleus retrofacialis (10.3%) which is known as a depressor-bradycardia-apnea response zone. This distribution was similar to that in the cat except that there were many labeled cells in the nucleus tractus solitarii of the cat (21.5%) but only a few in the rat (1.0%). The insignificant solitario-phrenic projection in the rat suggests a different organization of respiratory nuclei in this species.