The radiolabelled derivative of the potent and selective 5-HT3 receptor antagonist, GR65630, has been shown to label 5-HT3 receptors in homogenates of rat entorhinal cortex. We now report on the specific binding of this radioligand in homogenates of 16 discrete brain areas from several species and nine areas of human brain. We have further characterised specific binding to homogenates of rat area postrema and vagus nerve. In all species examined (rat, mouse, rabbit and ferret) the highest level of specific [3H]GR65630 (0.2 nM) binding was found in homogenates of the area postrema (26-83 fmol/mg protein). Binding in forebrain areas varied with species, Rat and mouse brain had relatively high levels of binding in cortical areas. The ferret and rat had relatively high levels in forebrain dopamine-containing areas. Of the areas of human brain examined the amygdala showed the highest level of specific [3H]GR65630 (0.2 nM) binding (3.0 fmol/mg protein). Specific [3H]GR65630 (0.05-2 nM) binding to homogenates of rat vagus nerve and area postrema was saturable (Bmax: vagus nerve 89.1 fmol/mg protein, area postrema 44.4 fmol/mg protein) and of high affinity (Kd: vagus nerve 0.50 nM, area postrema 0.24 nM). Selective 5-HT3 receptor agonists and antagonists potently inhibited [3H]GR65630 (0.2 nM) binding to homogenates of rat area postrema and vagus nerve. There was a close correlation between drug affinities in each area.