The distribution of binding sites for [3H]quipazine was examined in sections from rat brain. This radioligand has been demonstrated to label 5-HT3 receptors in membrane homogenate studies. Specificity of [3H]quipazine for these receptors was obtained by using 10(-7) M ICS 205-930, a highly selective 5-HT3 antagonist, to define non-specific binding. Several areas of dense 5-HT3-specific binding were detected in the medulla, most notably the nucleus of the solitary tract and the caudal portion of the spinal trigeminal tract. Low to moderate levels of 5-HT3 binding were seen in several forebrain regions, including the pyriform cortex, posterior nuclei of the amygdala, ventral tegmental area, anterior olfactory nucleus and superior colliculus. [3H]Quipazine autoradiography was also performed on brain sections from mouse, gerbil, hamster and guinea pig. Specific binding was quite low throughout most of the brains from these species; however, in all but the guinea pig, dense streaks of binding were detected in nucleus of the solitary tract (and in the mouse, the nucleus of the spinal tract of the trigeminal nerve). The distribution of 5-HT3 receptors in the brain may help explain some of the proposed CNS activities of 5-HT3-selective drugs. The anti-emetic and antinociceptive activities of 5-HT3 antagonists may be mediated by receptors in the sensory areas in the brainstem.