The histology and location of human respiratory chemosensitive fields are not known. In contrast, the physiology of respiratory chemosensory areas in the ventral medulla of cats has been studied extensively, and their anatomy has been partially described. Using basic principles of comparative cytoarchitecture and computer-aided reconstructions of serial-sectioned medullae, we describe the histology and three-dimensional distribution of putative respiratory chemosensors in the feline and human infant medulla. We found that ventrolateral neurons of the human nucleus conterminalis are homologous to neurons identified in the feline L chemosensitive field by Trouth and others, and that ventrolaterally situated neurons in the human arcuate nucleus correspond to neurons predominating in the feline S and M fields. In addition, there are foci of thickened marginal glia along the feline ventral medullary surface that colocalize with chemosensitive fields identified by physiologic studies reported by others; we identify similar foci in the infant medulla. Thickened marginal glia are intermixed with neuronal fibers, often adjacent to neurons of the feline chemosensitive fields and their human counterparts, suggesting that they constitute a chemosensory neuropil at the medullary surface. Computer-aided reconstructions provide insight into the three-dimensional topography of putative respiratory chemosensors and their relationships to other brainstem structures in ways not obvious in single or even multiple sections. This delineation of candidate human respiratory chemosensors is a first step toward their postmortem analysis in patients with central ventilatory control disorders where finding histological abnormalities in these sites would support their role in human ventilation.