We have examined the distribution of specific binding of the 5-HT3 receptor ligand [3H]GR65630 to rat brain using quantitative autoradiography. Brain tissue was frozen and thin sections (20 microns) cut. Slide-mounted tissue sections were incubated with [3H]GR65630 (0.2 nM) in the presence or absence of metoclopramide (30 microM, to define non-specific binding). After drying, sections were apposed to photographic film for 3 months. Films were quantified for radioactivity using image analysis. The highest level of specific binding was found in the area postrema (34.0 fmol/mg tissue). Specific binding was high in intermediate layers of the cortex, particularly the entorhinal, temporal and pyriform cortex (5.2-7.0 fmol/mg tissue). Amongst sub-cortical areas, binding was evident in the olfactory lobes, olfactory tubercle, hippocampus and basolateral amygdala (1.3-3.3 fmol/mg tissue). Specific binding was below measurable levels in other sub-cortical areas.