An experiment was conducted to assess the physiological and biochemical changes in relation to seed quality of fresh and aged onion seeds (Allium cepa L.). It was found that overall decrease in germination, vigour and enzyme activity with increase in ageing duration (data). Seeds aged artificially from 6 to 9 days showed lower germination percentage, seedling length and seedling vigour index, higher days to 50% germination, when compared to fresh and naturally aged seeds. Besides physiological changes, significant changes in biochemical parameters were also observed due to ageing. All the ageing treatments resulted in membrane damage, as evident from higher solute leakage in aged seeds. With respect to total soluble sugars in seed leachate, significant loss of soluble sugars was noticed. Impairment in the synthesis of enzymes, due to ageing was also noticed. Reduction to a greater extent in the enzyme activity, particularly dehydrogenase and alpha amylase activity was reduced due to seed ageing. There was large reduction in the synthesis of total soluble sugars and total soluble protein in aged treatments compared to fresh seed. The reason for loss in vigour both under natural and accelerated ageing appears unique but distinct in the rate at which they affect various parameters contributing to seed vigour.