Binding of the 5-HT3 receptor ligand [3H]BRL 43694 was investigated in the human medulla oblongata using in vitro autoradiography. High levels of saturable, displaceable binding (Bmax 1.88 pmol/mg protein, Kd 1.21 nM) were seen in the dorsal vagal complex but in no other medullary region. The results provide evidence for the existence of 5-HT3 receptor binding sites in a brain region involved in the control of vomiting in man.