Conditions have been defined which provide a means of selectively labeling subtypes of muscarinic receptors. The so-called M1 receptor population can be labeled with tritiated pirenzepine, while the receptor population labeled with tritiated quinuclidinyl benzilate QNB but not labeled with pirenzepine represents M2 receptor population. High and low affinity states of the receptors have also been defined on the basis of agonist displacement of antagonist binding. Both the M1 and M2 receptor populations undergo axonal transport and the affinity states of these receptors are altered by neurochemical and neurosurgical lesions. radioactive standards have been developed in this laboratory which provide a means of quantitating the femtomoles of receptor bound with each ligand in microscopic regions of the brain. The technology has also been devised to directly localize nicotinic cholinergic receptors using tritiated nicotine. It is now possible to localize several peptide receptors associated with cholinergic function including receptors for thyrotropin-releasing hormone TRH and somatostatin. The receptor autoradiographic technique has also been carried beyond the receptor level of localization by using compounds to label adenylate cyclase and the GTP binding protein. This methodology should provide an elegant means od determining how anticholinesterase exposure has affected these many parameters of cholinergic nerve function. Keywords Acetylcholine, Bioassay, Labeled substances.