The study of gene expression has been greatly facilitated by many recent advances in the technology of in situ hybridization histochemistry. The purpose of this report is to familiarize investigators with the basic principles of this technique and illustrate its potential with chosen examples in the field of neurobiology. A further goal is to provide insight into the specific options available in the selection of probes for use in non-radioactive (e.g. enzyme histochemical) detection of messenger RNA. Since many species differences should be apparent at the level of gene expression, this technology has considerable potential to aid in comparative studies of the nervous system.