Rapid biochemical tests developed for measuring viability of nondormant crop seed have been used for measuring dormant weed seed viability. The objective of this experiment was to propose and test a new method for validating one of these assays, the tetrazolium assay, for determining dormant weed seed viability. Subsamples of partially dormant jointed goatgrass seed from the same seed source were periodically subjected to sequential germination and tetrazolium assays as they afterripened until seed were completely nondormant. Changes in percent germinated, percent tetrazolium positive (presumably dormant), and percent tetrazolium negative (presumably dead) seed were graphed versus time, and compared with models of valid and invalid cases. The validation method showed that the tetrazolium assay accurately determined the viability of seed remaining after a 3-wk germination period for jointed goatgrass seed removed from joints at the start, but underestimated the viability for seed afterripened and germinated in joints. The validation method showed that additional research is needed to refine the tetrazolium assay for this latter purpose.