In the analysis of gene expression, the steady-state level of RNA transcripts is one of the most convenient parameters used to monitor the activity of an endogenous or introduced gene in cell lines and tissues. A variety of methods, such as S1 hybridization, RNase protection and Northern blotting, can be used to measure RNA levels. Which assay system is best depends largely on the type of information required, levels of sensitivity, and limitations of the particular in vivo system being examined. This article details the method for analyzing RNA by Northern blotting, which basically involves the isolation of RNA, its size fractionation by electrophoresis, transfer to a membrane, and detection by nucleic acid hybridization and autoradiography.