The cloning of the genes coding for many neurotransmitter receptors has made possible the development of probes for the visualization of the cells containing transcripts for these receptors at the microscopic level, using in situ hybridization histochemistry. The methodological aspects of the use of oligonucleotide probes for receptor mRNA visualization are discussed. The use of in situ hybridization to examine the cellular localization of receptor mRNAs, their relationship with receptor binding, receptor diversity and receptor mRNA regulation is illustrated with examples from our studies with glycine, muscarinic cholinergic, serotoninergic, and dopaminergic receptors.