Recently developed molecular biology techniques lend themselves to the study of both normal physiology and pathophysiology. This review discusses a number of such techniques and their applicability to the study of diabetes and hypertension with the renin-angiotensin system as an example. The demonstration of specific mRNA in tissue provides strong evidence that the protein for which it encodes is expressed there. The contribution of several methods to demonstrate mRNA expression including Northern analysis, slot blot analysis, solution hybridization, in situ hybridization, run-on assays, and polymerase chain reaction are discussed in the light of how these are used to study renin-angiotensin system mechanisms. Additionally, recent developments in the study of gene transfer and identification of genes are reviewed.