THE presence of cholinergic muscarinic receptors on neurones in many regions of the mammalian brain is known from classical electrophysiological studies, which showed that neuronal firing rates are altered by the microiontophoretic application of cholinergic muscarinic agonists and antagonists1. It is now possible to measure these receptors directly by biochemical procedures, some of which involve studying the responses of the cyclic guanosine monophosphate system in neuronal tissue to agonists and antagonists2–5. Other procedures utilised radiolabelled muscarinic agonists and antagonists that bind specifically and directly to presumed receptor sites6–12. One of these agonists13,14, 3-quinuclidinyl-benzilate (QNB) binds in a radiolabelled form to apparent muscarinic receptors in vitro6,7. Recent experiments indicate that it is also possible to demonstrate specific 3H-QNB binding to cholinergic muscarinic receptors in rat brain in vivo soon after intravenous administration15. Because of this latter factor, we have been able to examine the in vivo localisation of 3H-QNB in regions of rat brains by light microscopic autoradiography. We have found receptor sites associated with various groups of cells, some of which are known to be cholinoceptive. In general, the greatest densities of 3H-QNB sites were in telencephalic regions.