The distribution of FMRF-NH2-like immunoreactivity, visualized with immunohistochemical techniques, was plotted in a range from very dense to none. The rat area postrema had differential immunostaining with the greatest amounts occurring at its ventral and ventrolateral borders by the nucleus of the solitary tract. Immunoreactive cell bodies were located mainly in this region. Throughout the central region of the rat area postrema scattered immunostaining was consistently observed. The cat area postrema had a different, greater, and more complex pattern of immunostaining than the rat. Very dense to dense accumulations of immunostaining occurred in the ventromedial region of the area postrema bordering the solitary tract and dorsal vagal nuclei, while the central region possessed scattered amounts of immunoreactivity. Following colchicine treatment, no visible FMRF-NH2-like immunoreactive cell bodies were observed in the cat area postrema.