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Seed coat permeability is important to study as it plays significant roles in seed dormancy, germination, and protection from pathogens. Here we describe a commonly used seed coat permeability test known as the tetrazolium penetration assay with a method to quantify the levels of permeability. Tetrazolium red is a cationic dye that is widely used in seed viability testing. Tetrazolium salts are amphipathic cations, which, after penetrating the dead cells of the seed coat, are reduced to red-colored insoluble precipitates made up of formazans by active dehydrogenases (NADH-dependent reductases) in the embryo of seeds (Berridge et al., 1996). The intensity of red coloration is directly proportional to the permeability of the seeds. The quantification involves extraction of formazans from the incubated seeds and spectrophotometric determination of absorbance of formazan extracts at 485 nm. Note: This protocol is optimized for testing Arabidopsis thaliana seeds.