The biochemical phenotype or functional properties of a given cell can be determined by its pattern of gene expression. In turn, gene expression can be identified by measuring specific mRNA species within a cell using the powerful technique of in situ hybridisation histochemistry (ISHH). This involves the use of a labelled nucleotide probe with a sequence complementary to all or part of the sequence of target mRNA within the cell, and its hybridisation to that target mRNA. Furthermore, ISHH is a histochemical technique in which cell-to-cell relationships are maintained. Thus, macroscopic or microscopic localisation of the probe-mRNA hybrids in tissue sections can be used to precisely determine the cell type and structural organisation between different cells types expressing the gene(s) of interest. ISHH can be used to characterise neurones based on their expression of receptors for neurotransmitters, neuropeptides, synthetic enzymes for non-peptide neurotransmitters, signal transduction systems and structural proteins. ISHH allows the examination of characteristics and functional state of neurones and neuronal systems in response to external physiological and pathophysiological events, or in response to developmental signals during neurogenesis.