Abstract: Changes in respiratory rate, dry weight, available sugar content, activities of some respiratory enzymes and dehydrogenase activity in seeds of the Lombardy poplar (Populus nigra var. italica Dur.) were followed during storage with the view of elucidating factor(s) controlling seed viability. The following results were obtained. In general, the activities of all enzymes measured as well as the respiratory rate and sugar contents declined with the loss of viability of the seeds during storage. Specifically: (a) While the dry weight and available sugars decreased with the length of storage time, the quantities lost (5%–8%) was insignificant compared to the large amount (ca. 94%) of material still stored in the seeds at complete loss of viability. (b) Although the respiratory rate as well as the activities of amylase, peroxidase, catalase, cytochrome oxidase, ascorbic acid oxidase and polyphenol oxidase in the poplar seeds declined with time of storage, no close correlation could be seen between the course of decline in these activities and the fall in percentage of germination in the seeds. In fact, these activities persisted even long after the seeds had already lost their germination power. Of all the items studied in these experiments, only the course of decline in dehydrogenase activity (as measured by triphenyltetrazolium chloride reduction) exhibited a parallel relation with viability in the seeds (measured in terms of germination percentage). Dehydrogenase is entirely lost at the time of complete loss of germination power in these poplar seeds. When seeds had almost completely lost their viability, the addition of 2 mg/ml DPN or 0.1 mg/ml TPN significantly restored their dehydrogenase activity to that possessed by almost completely viable seeds (80%) germination. It is concluded from these experiments that the viability in these poplar seeds is apparently dependent on the state of the two pyridine nucleotides DPN and TPN in the seeds. The inactivation of these pyridine nucleotides during storage may be an important factor controlling the loss of poplar seed viability.