Abstract 1. In situ hybridization histochemistry permits the study of specific mRNAs of neuropeptides, enzymes involved in the synthesis of neurotransmitters, receptors and proteins associated with glial cells in nervous tissue. 2. The central and peripheral nervous systems are composed of heterogeneous elements and specific regulatory mechanisms occur in specific cells. 3. This review will focus on the localization and regulation of different mRNAs in the nervous system from Drosophila to human, as revealed by in situ hybridization histochemistry.