Sunflower (Helianthus annuus L.) seeds show more germination at high temperatures (25–30°C) than at temperatures below 20°C. Osmopriming with polyethylene glycol-6000 for 3–5 days at 15°C strongly increases germination at suboptimal temperatures. This stimulatory effect of priming persists after seed redrying and during subsequent storage at 20°C (55% RH) for at least 14 weeks. However, primed seeds deteriorate faster than untreated seeds during accelerated aging (45°C, 100% RH). The longer the priming treatment, the higher is the amount of germination but at the same time the higher is the sensitivity of seeds to accelerated aging. Priming enhances the respiratory activity of seeds transferred onto water and their ability to convert 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene. These effects remain after drying the seeds and are maintained in part during dry storage, whereas they disappear during accelerated aging. These results suggest that ACC-dependent ethylene production might be a good indicator of seed vigour; it increases with duration of priming and decreases very early during aging, well before significant loss of seed viability. Decrease in ACC conversion to ethylene indicates that aging is probably associated with membrane deterioration since in vivo ACC oxidase activity depends on membrane properties. However, no increase in electrolyte leakage is observed during aging.