In this study, the effects of NO (nitric oxide) on senescence occurring in the cotyledons of Helianthus annuus L. (sunflower) seedlings, were examined. Following germination of the sunflower the seedlings were grown in perlit for 10 days and then transferred into hydroponic culture containing 1/4 Hoagland solution. At day 23 approximately when senescence started occurring, 0.1, 1, 10, 100 and 400 mu M concentrations of Sodium nitroprusside prepared in 1/4 Hoagland solution used as NO donor were applied to the seedlings. The cotyledons of both control and experimental groups were harvested when the avarage green area in the control group reached 50%. Effects of NO in senescence process were determinated by measuring parameters such as Peroxidase activity, amount of chlorophyll and total soluble protein.