The effect of IAA and BA on the mechanism of senescence which occurs in the cotyledons of Helianthus annuus L. seedlings showing epigenic germination has been investigated. In this study 0, 01 mu M and 10 mu M IAA solutions were sprayed to the leaves and BA solutions with the same concentrations - to the cotyledons of 16 days-old seedlings. We have determined chlorophyll content, protein amount, peroxidase activity and the changes in the membrane permeability in the cotyledons of the treated and control seedlings. Compared to the control cotyledons IAA in the concentrations of 10 mu M and 0,01 mu M caused a breakdown of chlorophyll at the ratio of 64% and 54%, respectively. Soluble protein levels in the IAA at 10,mu M treatment decreased in the ratio of 12%. IAA treatments with 10 mu M and 0, 01,mu M stimulated POD activity with about 11 and 8 times, respectively. On the other hand, it has been observated that 10 mu M and 0,01 mu M BA treatment retarted senescence, whereas 0,01 mu M BA and 0,01 mu M IAA when were applied together enhanced senescence. It has been discussed that IAA which is known as an allosteric activators Of cytokinin oxidase stimulates the cytokinin oxidations so it increases senescence that occurs in the cotyledons. In this research, according to the results it has been thought that the senescence signal may be a substance in the nature of IAA.