Thetranscription and transport of messenger RNA (mRNA) are critical steps in regulating the spatial and temporal components of gene expression, butit has not been possible to observe the dynamics of endogenous mRNA in primary mammalian tissues. We have developed a transgenic mouse in which all -βactin mRNA is fluorescently labeled. We found that β-actin mRNA in primary fibroblasts localizes predominantly by diffusion and trapping as single mRNAs. In cultured neurons and acute brain slices, wefound that multiple -βactin mRNAs can assemble together, travel by active transport, anddisassemble upon depolarization by potassium chloride. Imaging of brain slices revealed immediate early induction of -βactin transcription after depolarization. Studying endogenous mRNA in live mouse tissues provides insight into its dynamic regulation within the context of the cellular and tissue microenvironment.